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A genetic analysis of the crooked toes defect in chickens Hollands, Keith Geoffrey 1956

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A GENETIC ANALYSIS OP THE CROOKED TOES DEFECT I N CHICKENS  by  Keith Geoffrey Hollands B.A., U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1 9 5 3 B.S.A., U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1954  A THESIS SUBMITTED I N PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF Master in  of Science  Agriculture  i n t h e Department o f Poultry We a c c e p t t h i s required  Science  t h e s i s as conforming  to the  standard  THE UNIVERSITY OF B R I T I S H COLUMBIA November, 1956  A GENETIC ANALYSIS OP THE CROOKED TOES DEFECT I N CHICKENS  ABSTRACT The an a b e r r a n t  c r o o k e d t o e s d e f e c t i n c h i c k e n i s a n example o f  polymorphic t r a i t  degeneration. property  The mechanism g o v e r n i n g  of balanced  heterozygosity.  associated with  inbreeding  i t s expression  i sa  genotypes based on o b l i g a t e l e v e l s o f  F i x a t i o n o f t h e t r a i t was a c c o m p l i s h e d i n  C a l i f o r n i a by a s e l e c t i o n program combined w i t h i n b r e e d i n g i n a S i n g l e Comb W h i t e L e g h o r n f l o c k .  The c r o o k e d t o e s  line  d e v e l o p e d f r o m t h i s p r o g r a m was c o m b i n e d , i n t h e p r e s e n t i n v e s t i g a t i o n , i n crosses with g e n e t i c a l l y unrelated S t r a i n and b r e e d  (New H a m p s h i r e ) c r o s s e s t o t h e  c r o o k e d t o e s l i n e were made r e c i p r o c a l l y a n d c a r r i e d t h e F-j^, F 2 a n d b a c k c r o s s  stock.  generations.  were mated f r o m t h e F^ g e n e r a t i o n on r e c i p r o c a l p a r e n t a l m a t i n g s .  through  O f f s p r i n g from each  to establish four lines  cross based  C o m p a r i s o n s were t h e n made t o  determine t h e l e v e l s o f i n c i d e n c e , t h e degree o f phenotypic expression, the association of the t r a i t e f f e c t s of sex-linkage, maternal  w i t h f i t n e s s and t h e  environment and i n b r e e d i n g .  C r o o k e d t o e s were f o u n d t o be a p o l y g e n i c  trait  c h a r a c t e r i z e d by semi-dominance and v a r i a b l e p e n e t r a n c e . trait  i s a s s o c i a t e d w i t h a number o f m o d i f i e r s a f f e c t i n g  Inbreeding  p e r se i n c r e a s e d i n c i d e n c e , and i n c r e a s e d  was a c c o m p a n i e d b y i n c r e a s e d e x p r e s s i v i t y . m a t i n g s between p a r e n t s  The fitness.  incidence  The b e h a v i o r o f  w i t h v a r y i n g degrees o f crooked t o e  i n c i d e n c e was u n p r e d i c t a b l e .  The a p p e a r a n c e o f t h e d e f e c t i s  determined t o a c o n s i d e r a b l e degree by t h e p r e s e n c e o f a p h y s i o l ogical threshold f o r i t s expression.  An u n s u c c e s s f u l a t t e m p t was  made t o l o c a t e m a r k e r g e n e s a s s o c i a t e d w i t h a g g r e g a t e s o f c r o o k e d toe determining  loci.  In presenting the  thesis in partial fulfilment  r e q u i r e m e n t s f o r an  of B r i t i s h it  this  freely  advanced degree a t the  Columbia, I agree t h a t  the  a v a i l a b l e f o r r e f e r e n c e and  agree that p e r m i s s i o n f o r extensive t h e s i s f o r s c h o l a r l y p u r p o s e s may o f my  D e p a r t m e n t o r by  stood that financial  be  s h a l l not  be  The  study.  I  copying of  of  University  of B r i t i s h  Columbia,  make  further this  g r a n t e d by  the  Head  I t i s underthesis  a l l o w e d w i t h o u t my  permission.  Department  shall  copying or p u b l i c a t i o n of t h i s gain  University  Library  his representative.  of  for  written  ACKNOWLEDGEMENTS  I would l i k e t o e x p r e s s to and J.  D r . Amp. P. H i c k s counsel Biely,  my deep a p p r e c i a t i o n  ( J r . ) f o r h i s continuous  throughout  the present  study.  assistance  To P r o f e s s o r  my s i n c e r e t h a n k s f o r h i s encouragement a n d  understanding. I am i n d e b t e d f a r m , B. E l l i s ,  to the staff  o f t h e UBC p o u l t r y  M. Hankey and D. F i s h e r , f o r  their  co-operation  and a s s i s t a n c e d u r i n g t h e c o u r s e  experimental  work and c o m p i l a t i o n o f t h e d a t a .  Finally, M. H o l l a n d s of  I would l i k e t o t h a n k J . H o l l i n g s and  f o r t h e i r valuable help  the manuscript  of the  and i l l u s t r a t i o n s .  i n the preparation  TABLE OP  CONTENTS  Page Abstract Introduction  1  L i t e r a t u r e Review  3  M a t e r i a l s and M e t h o d s  8  Results  13  CT I n c i d e n c e  13  Expressivity  29  Association with Fitness  35  Discussion  48  Summary  54  References Cited  55  L I S T OP  TABLES  Table I. II. III.  Page I n c i d e n c e o f CT  (UBC  production flocks)  M a t i n g d e s i g n o f t h e CT  experiment  Combined i n c i d e n c e o f CT averages i n the  9  and  11  expressivity  generation  14  IV.  I n c i d e n c e o f CT  i n t h e Fg g e n e r a t i o n  15  V.  I n c i d e n c e o f CT  i n the backcross matings  16  VI. VII. VIII.  I n c i d e n c e o f CT. Expressivity  Combined d a t a  o f CT  21  trait  30  C o m p a r i s o n o f CT i n c i d e n c e i n e m b r y o n i c d e a d w i t h hatched c h i c k s . F^ and F g e n e r a t i o n s 2  IX.  C o m p a r i s o n o f CT w i t h hatched  X.  i n c i d e n c e i n e m b r y o n i c dead  chicks.  H a t c h a b i l i t y and  Backcross matings  liveability.  F-^ and  39  Fg  generations XI.  H a t c h a b i l i t y and  37  42 liveability.  Backcross  m a t i n g s ..  44  L I S T OF FIGURES  Figure  Page 2  1.  White l e g h o r n c h i c k w i t h crooked toes  2.  I n c i d e n c e o f CT i n CT X SCWL m a t i n g s  17  3.  I n c i d e n c e o f CT i n CT X NH m a t i n g s  18  4.  Expressivity  i n r e c i p r o c a l CT X SCWL m a t i n g s  32  5.  Expressivity  i n r e c i p r o c a l CT X NH m a t i n g s  33  INTRODUCTION Crooked t o e s i n c h i c k e n and t u r k e y s i s a defect expressed  as a p e r s i s t e n t f l e x i o n  digits of the feet.  morphological  (Fig. l ) of the plantar  I n most c a s e s o n s e t o f t h e d e f e c t i s d u r i n g  t h e l a s t e i g h t d a y s o f i n c u b a t i o n b u t may o c c u r a t a n y t i m e p r i o r t o somatic  maturity  (Hicks, 1953).  Due t o r e d u c t i o n i n  growth o f t h e f l e x o r tendons o f the f e e t , t h e d i g i t s a r e unable to achieve  full The  extension.  genetic c h a r a c t e r i s t i c s of t h e crooked toes  complex  show t h a t i t i s a p o l y g e n i c a l l y d e t e r m i n e d t r a i t w h i c h o c c u r s i n many p o u l t r y f l o c k s a t a l o w l e v e l o f i n c i d e n c e 1949).  The t r a i t  i s amenable t o s e l e c t i o n .  a b i l i t y i n phenotypic incomplete  expression,  penetrance.  I t exhibits vari-  shows s e m i - d o m i n a n c e , a n d  The i n c i d e n c e o f t h e t r a i t  i n f l u e n c e d by environmental  i sgreatly  conditions.  Crooked toes i s s i m i l a r i n occurrence determination  ( H i c k s and L e r n e r ,  and genetic  t o a number o f t r a i t s i n o t h e r o r g a n i s m s w h i c h  have been c a t e g o r i z e d a s a b e r r a n t genetic determination  polymorphs.  The n a t u r e  of their  and wide d i s t r i b u t i o n i n p o p u l a t i o n s o f  c r o s s - f e r t i l i z i n g organisms i n d i c a t e s an a s s o c i a t i o n w i t h adapti v e p o t e n t i a l s i n those  populations.  characteristics i n populations  a t low frequencies  with disarrangements of balanced o r i e n t e d and m a i n t a i n e d The  The o c c u r r e n c e  ofthe  i s associated  g e n e t i c systems w h i c h a r e  by n a t u r a l s e l e c t i o n .  f o l l o w i n g s t u d y was d e s i g n e d  to enlarge  the under-  s t a n d i n g o f t h e g e n e t i c mechanisms c o n t r o l l i n g c r o o k e d t o e s . number o f s t r a i n a n d b r e e d c r o s s e s h a v e b e e n made t o o b t a i n f o r t h e F-^, F£, a n d b a c k c r o s s  generations.  A data  The i n c i d e n c e of.  c r o o k e d t o e s i s compared b e t w e e n d i f f e r e n t c r o s s e s a n d t h e e f f e c t s of g e n e t i c and non-genetic are  evaluated.  f a c t o r s that modify t h e i r  expression  Figure  1.  White l e g h o r n c h i c k crooked t o e s .  with  3 LITERATURE REVIEW During  the development of the s c i e n c e of  a number o f h e r i t a b l e t r a i t s u n d e r s e l e c t i o n had  little  h a v e b e e n r e p o r t e d whose  apparent connection Lerner  traits  ( D u b i n i n , 1948)  as p h e n o d e v i a n t s ; o t h e r s polymorphism.  i n v e s t i g a t i o n s was guinea  pig.  He  One  made by W r i g h t  behaviour  with a Mendelian  i n t e r p r e t a t i o n of i n h e r i t a n c e . them a s a b e r r a n t  Genetics  (1954) has  classed such  have  described  o f t h e more d e t a i l e d e a r l y ( 1 9 3 4 a ) on o t o c e p h a l y  d e s c r i b e d i t as a t r a i t  i n the  with variable manifest-  a t i o n i n v o l v i n g the r e d u c t i o n o f head p a r t s from a v e r y decrease i n the s i z e of the lower of c e p h a l i z a t i o n . incidence  I t was  jaw t o a v i r t u a l  p i g p o p u l a t i o n s and  by a number o f g e n e s w h i c h l a c k e d d o m i n a n c e . complete dominance of the t r a i t w h i c h r e a c h e d a 27% m u t a n t was  suppression  f o u n d t o be p r e v a l e n t a t a  (.05$$ i n g u i n e a  was  t o be He  slight  low determined  concluded  not p o s s i b l e .  I n one  that line  l e v e l o f i n c i d e n c e , he assumed a d o m i n a n t  present. W r i g h t (1934, b and  c) found a s i m i l a r  genetic  mechanism t o e x i s t i n t h e c o n t r o l o f P o l y d a c t y l y i n g u i n e a The  trait  was  e r i s e d by:  a l s o p r e v a l e n t a t a l o w i n c i d e n c e and was  genetic determination  pigs.  charact-  by a number o f g e n e s none o f  w h i c h showed d o m i n a n c e ; a v a r i a b l e r e s p o n s e i n c r o s s i n g t o d i f f e r e n t s t r a i n s ; the presence of p h y s i o l o g i c a l thresholds a f f e c t i n g i t s e x p r e s s i o n ; and with unfavorable polygenic t r a i t  environmental  increased incidence of conditions.  He  b e l i e v e d i t was  e x h i b i t i n g blending i n h e r i t a n c e which  a f f e c t e d by p h y s i o l o g i c a l t h r e s h o l d s .  r e p o r t e d by M u r r a y ( 1 9 3 2 ) ,  H o l t and  and  Wright (1946),  t h a t g e n e t i c c o n t r o l was number o f m o d i f i e r s . t o be  was  A s i m i l a r behaviour  P o l y d a c t y l y i n m i c e was  trait  expression  Chase (1951).  Holt  of  (1945),  Murray p o s t u l a t e d  d e t e r m i n e d by a d o m i n a n t gene and  a  On t h e o t h e r h a n d , C h a s e c o n s i d e r e d - the.  composed o f s e v e r a l g e n e s , none o f them d o m i n a n t ,  w h i c h were s t r o n g l y i n f l u e n c e d by a p h y s i o l o g i c a l t h r e s h o l d .  a  4  One o f t h e more e x t e n s i v e l y s t u d i e d g e n e t i c  complexes  i n D r o s o p h i l a m e l a n o g a s t e r i s t h a t o f a b n o r m a l abdomen 1952).  T h i s t r a i t was p r e s e n t  i n a Dichaete  (Sobel,  s t r a i n w h i c h he  e s t a b l i s h e d from a s t r a i n e x h i b i t i n g asymmetrical  abnormalities  o f t h e h e a d , t h o r a x , and abdomen d u r i n g t h e p u p a l  stage.  During  s e l e c t i o n f o r a b n o r m a l abdomen some i n b r e e d i n g was p r a c t i s e d . In  one s t r a i n ,  s e l e c t i o n was a b l e t o i n c r e a s e i n c i d e n c e f r o m  4.4% t o 28 - 3 1 % .  T h i s a p p a r e n t l y was t h e s e l e c t i o n l i m i t .  When  r e v e r s e s e l e c t i o n was p r a c t i s e d , no r e d u c t i o n i n i n c i d e n c e occurred.  I n another  Dichaete  s t r a i n , selection increased the  i n c i d e n c e t o 39$.  I n c i d e n c e was a f f e c t e d b y t e m p e r a t u r e a n d  time  When c r o s s e d i n t o o t h e r s t o c k s , t h e t r a i t  of hatching.  e x h i b i t e d i r r e g u l a r dominance. genes a r e p r e s e n t chromosomes.  as balanced  i s increased i n the  S e l e c t i o n c a n i n c r e a s e i n c i d e n c e when  i s present, u n t i l  this trait  t h a t a b n o r m a l abdomen  p o l y g e n i c systems i n v o l v i n g s e v e r a l  The e x p r e s s i o n o f t h e t r a i t  presence of Dichaete. Dichaete  He c o n c l u d e d  homozygosity o f t h e genes  determining  i s reached. A n o t h e r example i n D r o s o p h i l a i s p o d o p t e r a  Hannah, a n d P i t e r n i c k , 1 9 5 1 ) . and  This t r a i t  has a p o l y g e n i c  i s f o u n d a t a l o w i n c i d e n c e i n many s t r a i n s .  of t h i s t r a i t  The  i s s i m i l a r t o t h a t o f a b n o r m a l abdomen.  (1949) g i v e s e v i d e n c e t a i l abnormality,  of a complicated  kinky t a i l ,  (Goldschmidt, basis  behaviour Donald  genetic structure f o r a  i n pigs that i s apparently the  same i n i t s m a j o r f e a t u r e s a s t h e a b o v e t r a i t s .  There a r e a l s o  some i n t e r e s t i n g c h a r a c t e r s i n p l a n t s s u c h a s r o g u e p e a s and  (Bateson  P e l l e w , 1 9 2 0 ; R e n a r d , 1 9 3 0 ) whose g e n e t i c . s t r u c t u r e may be  r e l a t e d t o t h e above t r a i t s Two i m p o r t a n t  described i n animals.  s t u d i e s o f t h i s type o f c h a r a c t e r i s t i c ,  i n w i d e l y d i v e r g e n t m a t e r i a l , have r e s u l t e d i n e s s e n t i a l l y t h e same i n d e p e n d e n t l y data.  The f i r s t  formulated  interpretation of  o f t h e s e s t u d i e s was c o m p l e t e d b y D u b i n i n  on e x t r a w i n g v e n a t i o n i n D r o s o p h i l a . to  experimental  The t r a i t  (1948)  was d e m o n s t r a t e d  be w i d e l y d i s t r i b u t e d i n a number o f w i l d p o p u l a t i o n s o f  D. m e l a n o g a s t e r and t h r e e o t h e r s p e c i e s o f D r o s o p h i l a .  Although  5  a seasonal  cycle o f f l u c t u a t i o n i n incidence occurred, i t  remained a t an average l o w l e v e l o f 0.29$.  S e l e c t i o n and  i n b r e e d i n g were e f f e c t i v e i n i n c r e a s i n g t h e i n c i d e n c e and degree o f e x p r e s s i o n o f t h i s t r a i t .  The m a j o r i t y o f t h e  s e l e c t e d l i n e s showed t h e t r a i t w i t h i n f i v e g e n e r a t i o n s one  l i n e i t d i d not appear u n t i l t h e twenty-second generation o f  b r o t h e r - s i s t e r matings.  When l i n e s were  i n t h e o f f s p r i n g were u n p r e d i c t a b l e . c h a r a c t e r i s e d by p l a s t i c and  but i n  a polygenic  crossed, the incidences  G e n e t i c a l l y , t h e t r a i t was  semi-dominance, incomplete  system o r g a n i s e d  penetrance,  i n t o gene b l o c k s w i t h i n t h e  s e c o n d chromosome. The (CT)  s e c o n d i n v e s t i g a t i o n was on t h e  crooked toes  (1953), and H i c k s a n d L e r n e r  i n c h i c k e n by H i c k s  defect  (1949).  T h i s t r a i t h a s t h e same e s s e n t i a l f e a t u r e s a s t h a t o f e x t r a w i n g venation.  I t i s widespread a t a l o w i n c i d e n c e and e x p r e s s i o n i n  poultry flocks; with selection thet r a i t  c a n be f i x e d ;  inbreeding  i n t h e a b s e n c e o f s e l e c t i o n w i l l i n c r e a s e CT i n c i d e n c e ; i t i s variable i nexpression,  s o much s o t h a t t h e r e s u l t s o f i n d i v i d u a l  m a t i n g s a r e u n p r e d i c t a b l e ; a s i n c i d e n c e o f CT i n c r e a s e s , s o d o e s the degree o f e x p r e s s i o n ; and i t i s a p o l y g e n i c t r a i t i s e d by a v a r y i n g t h r e s h o l d f o r e x p r e s s i o n , incomplete and  incomplete The  t h e o r e t i c a l i n t e r p r e t a t i o n of the r e s u l t s o f these by Lerner  (1954)  i n c o r p o r a t e d t h e i n t e r p r e t a t i o n s o f Dubinin and Hicks  l a r g e r a n d more s p e c u l a t i v e t h e o r y theory has  dominance,  penetrance.  experiments has r e c e i v e d i t s f u l l e s t treatment who  character-  of genetic homeostasis.  assumes t h a t a p o p u l a t i o n o f c r o s s - f e r t i l i z i n g  into a This  organisms  s e l f - r e g u l a t i n g p r o p e r t i e s which enable i t t o balance i t s  genetic composition  and t o r e s i s t  sudden c h a n g e s i n environment.  These p r o p e r t i e s a r e n o t s i m p l y t h e s u b s t i t u t i o n a t i n d i v i d u a l loci  o f l e s s f a v o r a b l e a l l e l e s b y more f a v o r a b l e ones b u t i n c l u d e  the i n t e g r a t i v e a b i l i t i e s  o f a p o p u l a t i o n b y s h i f t i n g gene  f r e q u e n c i e s a n d gene a r r a n g e m e n t s t o adapt t o c h a n g i n g conditions. The  environmental  These p r o p e r t i e s have e v o l v e d u n d e r n a t u r a l s e l e c t i o n .  g e n e t i c mechanism w h i c h b e s t f i t s t h i s t h e o r y i n v o l v e s a n  6  an obligate degree of heterozygosity i n most individuals. In support of t h i s theory, Lerner has drawn upon several other important l i n e s of investigation that have been instrumental i n demonstrating genetic properties i n populations that transcend those i n individuals. Waddington (1942) has demonstrated that c r o s s - f e r t i l i z i n g organisms have the power of regulating their development (canalization) which produces a uniformity of phenotypic expression i n a population i n spite of genetic v a r i a b i l i t y between individuals. The action of natural selection i s to favor the average phenotype ( s t a b i l i z i n g selection) with the elimination of extreme phenotypes (Schmalhausen, 1949). Heterozygosity best explains greater f i t n e s s i n the average individual by "regulating his ontogeny to remain within the normal bounds of development. Organised and balanced systems of polygenes have been described by Mather (1941, 1943, and 1953; Wigan-and Mather, 1942; Wigan, 1944) which require a heterogeneous condition f o r optimum fitness i n natural populations. They are based on two types of balancing mechanisms, both controlled by natural selection. Internal balance refers t o the organisation of genes along a chromosome. Crossing over and chromosomal aberrations such as inversions (Dobzhansky, 1947a) w i l l develop and maintain desirable gene arrangements within a chromosome. Relational balance refers to interactions between homologous chromosomes and between chromosomes. This should be the most important balancing mechanism and i s demonstrated repeatedly when inbred individuals exhibit greater phenotypic v a r i a b i l i t y than crossbred individuals. An extensive study by Mather and Harrison (1949) who selected f o r high and low chaetae number i n Drosopila melanogaster. indicated that selection with inbreeding p s e t s the genetic balance of polygenic systems and can be restored either by relaxing selection or by the adjustment of the polygenic systems to a new balance. u  The theory with regard to phenodeviants rests on t h e i r properties as balanced polygenic complexes. Most individuals i n a population possess balanced genotype and are, therefore,  7  p h e n o t y p i c a l l y normal.  N e v e r t h e l e s s , a few i n d i v i d u a l s i n  every g e n e r a t i o n f a l l below a t h r e s h o l d of o b l i g a t e h e t e r o z y g o s i t y and t h r o u g h u n b a l a n c e o f t h e p o l y g e n i c s y s t e m a r e u n a b l e to r e g u l a t e themselves along a normal developmental  pattern.  8  MATERIALS AND METHODS B i r d s f r o m t h r e e l i n e s o f s t o c k were u s e d i n t h e experiment.  A s m a l l number o f f e r t i l e  eggs f r o m t h e U n i v e r s i t y  o f C a l i f o r n i a CT l i n e were s h i p p e d t o t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a i n t h e e a r l y s p r i n g o f 1954.  T h i s l i n e was d e r i v e d  from  t h e S i n g l e Comb W h i t e L e g h o r n (SCWL) p r o d u c t i o n f l o c k a t t h a t institution. 1950  The CT l i n e h a s b e e n a p u r e CT b r e e d i n g l i n e  since  ( o n e c h i c k o u t o f 146 was c l a s s i f i e d a s n o r m a l a t t i m e o f  hatch i n 1953). The  s e c o n d l i n e came f r o m t h e UBC SCWL p r o d u c t i o n  flock.  T h i s s t r a i n h a s b e e n e s t a b l i s h e d f o r o v e r 20 y e a r s a n d h a s b e e n e s s e n t i a l l y c l o s e d s i n c e t h e n a s f a r a s c a n be d e t e r m i n e d . f l o c k i s r e p r o d u c t i v e l y i n e f f i c i e n t and a mediocre  The  producer.  S e l e c t i o n i n t h e SCWL f l o c k h a s b e e n t o w a r d i m p r o v e m e n t o f t h e p r o d u c t i o n i n d e x w i t h some s e l e c t i o n p r e s s u r e  f o r egg s i z e .  t h i r d l i n e came f r o m t h e UBC New H a m p s h i r e (NH) p r o d u c t i o n w h i c h was d e v e l o p e d  from a combination  early mid-forties.  The NH f l o c k h a s b e e n d e v e l o p e d  purpose s t r a i n , w i t h approximately for  flock  of several strains i nthe as a dual  equal s e l e c t i o n pressure  t h e i m p r o v e m e n t o f b o t h meat a n d e g g p r o d u c t i o n .  reproductively  The  applied  The f l o c k i s  efficient.  T h e r e was a v e r y l o w i n c i d e n c e o f CT ( T a b l e I ) i n t h e UBC  p r o d u c t i o n f l o c k a t t h e time  that the t r a i t  was r e c o r d e d .  of hatch i n the f i r s t two years  Nevertheless,  i n t h e 1953-54 s e a s o n  t h e r e was a s i g n i f i c a n t l y g r e a t e r i n c i d e n c e i n t h e SCWL f l o c k t h a n i n t h e NH f l o c k . ing  The d i f f e r e n c e s a r e r e d u c e d i n t h e f o l l o w -  s e a s o n a l t h o u g h t h e SCWL f l o c k a g a i n h a s a h i g h e r  o f CT.  incidence  W h i l e t h e e f f e c t s o f t h e two s e l e c t i o n p r o g r a m s a r e  difficult  t o assess  the broader  r e g a r d i n g t h e i r e f f e c t s on t h e CT c o m p l e x ,  h e t e r o g e n e i t y and the c a n c e l l i n g e f f e c t s o f a d u a l  s e l e c t i o n p r o g r a m i n t h e NH f l o c k may c o n c e i v a b l y l e a d t o a b e t t e r balanced  flock  genotype.  T h e r e w e r e no CT b i r d s p r e s e n t  a t h a t c h i n t h e dam  f a m i l i e s f r o m w h i c h b i r d s were t a k e n t o mate w i t h t h e CT l i n e .  A l l m a t i n g s made i n t h e c o u r s e  California  o f the experiment a r e  9  Table I I n c i d e n c e o f CT among h a t c h e d  chicks i n the  U n i v e r s i t y o f B r i t i s h Columbia p r o d u c t i o n  flocks.  Season  Breed  Chicks hatched  CT a t h a t c h  1953- 54  SCWL  1547  29  1.87 **  NH  1518  10  0.66 **  SCWL  1470  4  0.27  NH  1482  0  0-.00  1954- 55  **  % incidence  The d i f f e r e n c e i n CT i n c i d e n c e b e t w e e n b r e e d s i s s t a t i s t i c a l l y s i g n i f i c a n t a t t h e .01$ l e v e l .  10  shown i n T a b l e I I . T h r e e b r e e d i n g p e n s w e r e made up w i t h t h e parental birds i n the f a l l one o f t h e l i n e s  o f 1954, each headed  used i n t h e experiment.  by a s i r e  from  Each pen c o n t a i n e d  f i v e dams ( o n l y f o u r were a v a i l a b l e f r o m t h e CT l i n e ) f r o m of t h e t h r e e l i n e s .  each  Where p o s s i b l e dams w i t h i n a p e n were  u n r e l a t e d b u t e a c h dam h a d f u l l  s i b i n e a c h o f t h e o t h e r two p e n s .  W i t h i n a mating pen comparison o f t h e r e s u l t s  between  m a t i n g c a t e g o r i e s was b a s e d o n t h e d i f f e r e n c e b e t w e e n h a l f - s i b groups.  R e c i p r o c a l matings o c c u r r e d between  breed crossed.  each s t r a i n and  Thus g e n o t y p i c d i f f e r e n c e s were m i n i m i z e d  between  m a t i n g c a t e g o r i e s p e r m i t t i n g t h e maximum e f f e c t o f t h e CT  complex  t o be e x p r e s s e d . ion  The m a t i n g d e s i g n a l s o w o u l d p e r m i t t h e d e t e c t -  o f s e x - l i n k a g e and maternal- e f f e c t s i f t h e s e c h a r a c t e r i s t i c s  were p r e s e n t . The CT l i n e dams were s o c i a l l y  dominated by t h e o t h e r  b i r d s i n t h e p a r e n t a l matings t o such an e x t e n t t h a t they had t o be h o u s e d s e p a r a t e l y i f t h e y were t o s u r v i v e .  Similarily, the  NH dams d o m i n a t e d t h e CT s i r e s u f f i c i e n t l y t o p r e v e n t s u c c e s s f u l matings.  C o n s e q u e n t l y , t h e t h r e e s i r e s were housed i n i n d i v i d u a l  c a g e s a n d t h e p a r e n t a l m a t i n g s were c o n d u c t e d w i t h a n a r t i f i c a l insemination  program.  In the f a l l  o f 1 9 5 5 , f o u r b r e e d i n g p e n s were made up  w i t h F ^ b i r d s , one f r o m e a c h o f t h e r e c i p r o c a l m a t i n g s o f t h e CT l i n e w i t h t h e SCWL l i n e a n d t h e CT l i n e w i t h t h e NH l i n e . I I i n d i c a t e s t h e phenotype and t h e l i n e  of origin  Table  of the birds  in  each b r e e d i n g pen.  I t w i l l be s e e n t h a t t h e m a t i n g  structure  of  e a c h p e n was s i m i l a r t o t h a t i n t h e p a r e n t a l m a t i n g , t h a t i s ,  e a c h m a l e was mated w i t h t h r e e c a t e g o r i e s o f f e m a l e s w i t h a maximum o f f i v e dams i n e a c h c a t e g o r y .  I n these matings  there  were t h r e e s h i f t s o f m a l e s t o e a c h p e n , so t h a t e a c h dam was mated t o a n F-^ s t r a i g h t t o e ( S T ) s i r e , a n F ^ CT s i r e , a n d a f r o m one o f t h e p a r e n t a l l i n e s .  sire  N o t e t h a t t h e b a c k c r o s s P-^ CT  s i r e i n g r o u p 12 ( T a b l e I I ) i s t h e same s i r e a s i h a t i n g r o u p 1 4 .  Table I I M a t i n g d e s i g n o f t h e CT e x p e r i m e n t .  B e n e a t h eaeh s i r e  i s t h e d e s i g n a t i o n o f each o f  the  t h r e e c l a s s e s o f dams w i t h w h i c h he was m a t e d .  I n t h e F-^ m a t i n g s , t h e p a r e n t s o f  the  b i r d s a r e i n d i c a t e d by t h e numeral and l e t t e r i n p a r e n t h e s i s .  Parental matings; 1.  CT cf  a . CT  2. °-¥  SCWL cf  a. CT  3.  °4  NH o"  a . CT  n  b. SCWL 8°-  b. SCWL $ $  b. SCWL ? ?  c . NH  c. NH  c. NH  n  n  W  P^ m a t i n g s : 4^  P~"ST cf ( l . b )  a. P b. P c. P  1  1  x  5.  ST cf  x  ST  x  ST o" ( 3 . a )  (l.c)  a. P  1  ST $ ? ( 3 . a )  (l.c)  b. P  1  CT ? ? ( 3 . a )  c. P  1  CT $$ ( l . a )  x  CT o* ( 3 . a )  CT  b. P _ CT ? ? (2.a.)  b. P  1  CT  c. P  c. P  X  NH °-¥-  (l.b)  SCWL  (2.b)  CT cf  4a, b, and c.  ±  ]  9.  P  1  CT ? ? (l.a)-,^  1  CT cf ( 2 . a )  Dams:- same a s i n  10.  P-L SCWL c*  Dams:- same a s i n 5a, b, and c.  x  (3.c)  CT o* ( l . c )  Dams:- same a s i n  5a, b, and c. 13.  P  (l.c)  P  a. P  Dams:- same a s i n  •1.  1  ST $$- ( 2 . a )  4a, b, and c. 1  P  a. ¥  Dams:- same a s i n  P  6.  ST ? ? ( l . b )  8. P.^ CT cf ( l . b )  12.  P-L ST cf ( 2 . a )  7.  11.  Dams:- same a s i n  6a, b, and c. 14.  P-L CT cf  Dams:- same a s i n 6a, b, a n d c.  P  7a, b, and c. 15.  P-j_ NH cf  Dams:- same a s i n 7a, b, and c.  I n m a t i n g 5.c. one o f t h e f e m a l e s i s t h e dam o f t h e m a l e s w i t h w h i c h s h e i s m a t e d .  12  The  i n c u b a t e d eggs w h i c h f e i l e d t o h a t c h w e r e b r o k e n  open t o d e t e r m i n e t h e c a u s e o f d e a t h .  I n t h o s e embryos w h i c h  d i e d a f t e r t h e t w e l f t h day o f i n c u b a t i o n , t h e s e x and o r absence  o f CT was  recorded.  Day  presence  o l d c h i c k s were p e d i g r e e d  and p r e s e n c e a n d g r a d e o f CT was r e c o r d e d .  A t 10  weeks o f  a l l b i r d s were a g a i n h a n d l e d r e c o r d i n g s e x and p r e s e n c e g r a d e o f CT.  and  I n t h e c r o s s e s t o t h e 1JH l i n e a n a t t e m p t was  t o i d e n t i f y c o l o r genes w h i c h might p a r t o f t h e CT c o m p l e x  made  i n d i c a t e l i n k a g e between a  and a s i n g l e d o m i n a n t  gene.  Birds dying  b e f o r e t h e t e n t h week were a u t o p s i e d and c a u s e o f d e a t h and determined.  age,  sex  1  13  RESULTS  CROOKED TOE INCIDENCE CT  '  i n c i d e n c e h a s been measured by c o m b i n i n g t h e  i n c i d e n c e among e m b r y o n i c d e a d ( 1 3 - 2 1 d a y s o f i n c u b a t i o n ) a n d hatched c h i c k s .  Embryos w h i c h d i e d b e f o r e  t h i r t e e n days o f  i n c u b a t i o n were n o t i n c l u d e d b e c a u s e t h e d e v e l o p m e n t o f t h e CT c h a r a c t e r i s not expressed  before t h a t time (Hicks 1953).  combined i n c i d e n c e i s p r o b a b l y  t h e most r e l i a b l e i n d i c a t o r o f  t h e t r u e g e n e t i c p i c t u r e s i n c e i t i n c l u d e s more z y g o t e s e i t h e r t h e e m b r y o n i c dead o r h a t c h e d c h i c k s a l o n e . to hatching time the zygotes uniform  The  than  F u r t h e r , up  h a v e b e e n l i v i n g u n d e r t h e most  external conditions possible.  Thus d a t a o b t a i n e d  t h i s period of the l i f e cycle a r e l e s s subject t o  during  environmental  modifications. The  i n c i d e n c e o f CT i n t h e p a r e n t a l l i n e s a n d i n t h e F^,  F^ a n d b a c k c r o s s F i g s . 2 a n d 3.  generations  i spresented  i n Tables  I I I - V and  The d a t a , s u m m a r i z e d a t t h e s i r e f a m i l y  level,  i l l u s t r a t e s one o f t h e i n h e r e n t d i f f i c u l t i e s a s w e l l a s one o f t h e important  features c h a r a c t e r i s t i c of phenodeviants.  This  feature  i s t h e extreme v a r i a b i l i t y between g r o u p s i n t h e e x p r e s s i o n o f t h e CT c h a r a c t e r .  The v a r i a b i l i t y i s e m p h a s i z e d when t h e d a t a  a r e e x a m i n e d a t t h e dam f a m i l y l e v e l . i n t h e Fg g e n e r a t i o n  A t y p i c a l example o f t h i s  i s d e m o n s t r a t e d i n t h e s i r e f a m i l y F ^ CT cT X  f r o m t h e p a r e n t a l m a t i n g CT o* X NH  F-j^ ST  In this  family  o f f i v e dams, t h e CT i n c i d e n c e r a n g e d f r o m 1 4 . 7 $ i n t h e p r o g e n y o f t h e dam f a m i l y w i t h t h e l o w e s t dam  family with the highest  i n c i d e n c e o f CT t o 5 3 - 3 %  incidence.  i n the  C o n s e q u e n t l y , i t h a s been  d e s i r a b l e t o combine t h e d a t a i n t o l a r g e groups ( T a b l e V I ) f o r analysis.  The f o u r p h e n o t y p i c  same p a r e n t a l l i n e  c l a s s e s i n t h e F^ m a t i n g s f r o m t h e  (Table IV) i l l u s t r a t e a l a c k o f correspondence  between t h e  phenotype o f t h e parents  and t h e phenotype o f t h e  offspring.  I n t h e m a t i n g o f F ^ ST b i r d s , t h e o f f s p r i n g h a d a CT  i n c i d e n c e o f 1 8 . 2 $ w h i l e t h a t o f F ^ CT b i r d s h a d a CT i n c i d e n c e of 12.4$.  The r e c i p r o c a l F ^ CT X ST m a t i n g s p r o d u c e d o f f s p r i n g  w i t h a CT i n c i d e n c e o f 24 - 2 5 $ . S i m i l a r c o m p a r i s o n s b e t w e e n the F  phenotypic  c l a s s e s d e r i v e d from t h e other p a r e n t a l  lines  Table I I I Combined i n c i d e n c e * o f CT a n d e x p r e s s i v i t y a v e r a g e s i n t h e P-^ generation;  CT Dams  o f c r o s s e s b e t w e e n CT, SCWL a n d NH l i n e s .  sire  SCWL s i r e  No. CT/ total  NH s i r e  No. CT/  No. CT/  <fo  Expressivity  total  fo  Expressivity  total  fo E x p r e s s i v i t y  CT  42/46  91.3  3.65  30/52  57.7  1.70  7/40  17.5  1.15  SCWL  33/178  18.5  1.12  20/195 10.2  1.01  3/121  2.5  1.00  5/178  2.8  1.01  1.00  2/80  2.5  1.00  NH  0/175  0.0  * Combined i n c i d e n c e = i n c i d e n c e among d e a d embryos ( 1 3 - 21 d a y s o f i n c u b a t i o n ) + i n c i d e n c e among h a t c h e d c h i c k s .  H  15  Table IV I n c i d e n c e o f CT i n t h e  generation i n reciprocal  c r o s s e s o f t h e CT l i n e w i t h UBC SCWL a n d NH l i n e s .  CT X SCWL l i n e s  CT o*X UBC No. CT/ total  l  F  l  *1  P  l  Expressivity  UBC cf X CT ffi No. CT/ total  Expressivity  Dams  Sire F  ffi  ST X F  1  ST  3 4 / 9 3  36.6  1 . 8 7  3 3 / 1 1 4  29.0  1.57  ST X F  1  CT  2 0 / 5 4  37.0  1.65  1 9 / 8 4  22.6  1.36  CT X F  X  ST  5 0 / 1 0 2  4 9 . 0  1.54  5 5 / 9 8  5 6 . 1  2 . 1 4  4 3 / 7 1  60.6  2 . 0 7  4 5 / 9 7  4 6 . 4  1.96  CT X F - L CT  CT X NH l i n e s  P  l  F  l  F  l  F  l  ST X F  1  ST  2 2 / 1 2 1  1 8 . 2  1.25  5 3 / 1 6 1  32.9  1 . 6 2  X  CT  2 1 / 8 6  24.4  1 . 4 4  4 / 3 7  1 0 . 8  1.20  1  ST  4 0 / 1 5 9  25.2  1 . 1 8  57/147  38.8  1 . 8 1  X  CT  1 1 / 8 9  12.4  1.15  7/27  25.9  1.61  ST X F CT X F CT X F  16  Table V I n c i d e n c e o f CT i n t h e b a c k c r o s s m a t i n g s o f P birds with  CT X SCWL l i n e s  the parental  CT cf X UBC ffi UBC c/X CT °-$ No. CT/ No. CT/ total % Expressivity total $ Expressivity  P p  l  F  l  lines.  C T  x  cf  ^  SCWL  cf  ST  %%  70/93  75.3  2.81  14/85  16.5  1.12  CT  n  59/60  98.3  3.61  25/97  25.8  1.12  P-L S T cf p  l  SCWL  ? ?  1/109 P  p  l  CT  P  l  P  l  SCWL  ? ?  X NH ST  CT  $ $  17/84  P  l  P  l  NH  W  l  P  l  C T  1.09  P  cf  8.2  1.03  21/23  91.3  2/21  9.5  1.19  1  10/113  S T  cf  8.8  F  S T  ±  ? ?  10/110  ±  C T  cf  9.1  cf  1.08 5/6  NH ? ?  NH cf  11/134  CT  CT  1  90.0  ¥  P  x  cf  72/80  P P  C T  1  20.2  lines  1.02  0.9  83.3 P  x  C T  4.00  cf  1.06 25/25  100  3.55  Figure 2.  Incidence of CT i n reciprocal CT X SCWL matings. Recorded i n percent.  18  Figure 3.  Incidence of CT i n reciprocal CT X NH matings. Recorded i n percent.  19  show a l a c k o f c o r r e s p o n d e n c e b e t w e e n p a r e n t - o f f s p r i n g p h e n o t y p e s . The  P  1  i n c i d e n c e o f CT i n t h e CT X SCWL c r o s s e s i s  i n t e r m e d i a t e between t h e  t w o p a r e n t a l l i n e s b u t a p p r o a c h e s more  c l o s e l y t h e i n c i d e n c e i n t h e SCWL l i n e .  A similar  e x i s t s i n t h e combined r e c i p r o c a l c r o s s e s  situation  i n t h e CT X NH l i n e s  where t h e i n c i d e n c e i n t h e F ^ p r o g e n y i s e v e n more  closely  a s s o c i a t e d w i t h t h a t e x i s t i n g i n t h e ST p a r e n t s . When t h e P^ b i r d s were mated i n t e r s e , t h e i n c i d e n c e o f CT  increased i nboth the s t r a i n  crosses.  Although  (CT X SCWL) a n d Ithe b r e e d  (CT X NH)  t h e Pg i n c i d e n c e i n t h e p r o g e n y f r o m t h e SCWL  l i n e s r e m a i n e d h i g h e r t h a n i n t h e NH l i n e s ,  t h e r e was p r o p o r t i o n -  a t e l y a much g r e a t e r i n c r e a s e i n t h e l a t t e r  group.  In the backcross  m a t i n g s o f t h e P^ p r o g e n y t o t h e  p a r e n t a l l i n e s , progeny o f these matings very n e a r l y approach t h e CT  i n c i d e n c e i n t h e p a r e n t a l l i n e t o w h i c h t h e y were m a t e d .  This  t e n d e n c y i s c a r r i e d t o t h e g r e a t e s t e x t r e m e i n t h e CT X NH P^ b i r d s backcrossed  t o t h e CT p a r e n t a l l i n e .  I n t h i s case t h e back-  c r o s s p r o g e n y e q u a l t h e i n c i d e n c e o f c r o o k e d t o e s i n t h e CT l i n e . Sex-Linkage E v i d e n c e o f s e x - l i n k a g e i n t h e CT c h a r a c t e r was CT was  indicated f o r several generations l i n e i nC a l i f o r n i a .  d u r i n g the development o f t h e  No e v i d e n c e s u g g e s t i n g  found i n t h e present  ( H i c k s , 1953)  this  relationship  experiment.  Maternal E f f e c t s An (Table VI)  examination  o f t h e d a t a between r e c i p r o c a l m a t i n g s  i n t h e c r o s s e s t o t h e SCWL a n d t o t h e NH l i n e s  t h a t i n t h e P-^ g e n e r a t i o n  shows  t h e r e i s a much h i g h e r CT i n c i d e n c e i n  t h e m a t i n g s t o CT l i n e dams t h a n i n t h e m a t i n g s t o CT l i n e The CT and 1  sires.  higher incidence i ss t a t i s t i c a l l y s i g n i f i c a n t both i n the X SCWL c r o s s e s  ( c h i s q u a r e = 2 8 . 7 0 9 , P = _ .01 w i t h 1  i n t h e CT X NH c r o s s e s  d.f.)  ( c h i s q u a r e = 1 0 . 8 7 4 , P = / .01 w i t h  d.f.). When t h e two c o m p o n e n t s , e m b r y o n i c d e a d and''hatched  c h i c k s a r e examined s e p a r a t e l y , t h e y b o t h e x h i b i t a h i g h e r  20  i n c i d e n c e o f CT i n t h e c r o s s e s t o t h e CT l i n e dams ( s e e b e l o w ) . CT i n c i d e n c e i n e m b r y o n i c d e a d . F_ g e n e r a t i o n . SCWL ( f X CT ? ? CT  cf X SCWL ? ?  14/17 21/35  82.3$ 60.0$  CT i n c i d e n c e i n h a t c h e d  5/6  NH cf X CT $$CT cf X NH  chicks.  83.3$  3/13  23.1$  F-^ g e n e r a t i o n .  SCWL cf X CT ? ?  16/35  4 5 . 7 $ **  NH cf X CT  2/34  5.9$  CT  12/143  8.4$ **  CT cf X NH  2/163  1.2$  cf X SCWL $ ?  ** S i g n i f i c a n t a t 1 $ l e v e l Failure t o obtain s t a t i s t i c a l l y m a j o r i t y o f comparisons"  i s due t o t h e s m a l l number o f p r o g e n y i n  the separate c a t e g o r i e s . i s a strong maternal toward  significant differences i n the  The e v i d e n c e  l e a v e s no d o u b t t h a t t h e r e  e f f e c t c o n t r i b u t e d b y dams f r o m t h e CT l i n e  t h e e x p r e s s i o n o f t h e CT c h a r a c t e r i n t h e F ^ g e n e r a t i o n .  A h i g h d e g r e e o f i n b r e e d i n g i n t h e CT l i n e the cause o f t h e m a t e r n a l  effect.  This could occur provided the  CT l i n e dams were g e n e t i c a l l y u n b a l a n c e d producing unfavorable  ( F = .55) i s p r o b a b l y  ( r e l a t i v e l y homozygous)  environmental conditions w i t h i n t h e egg.  I n t h e F2 g e n e r a t i o n , p r o g e n y d e s c e n d e n t f r o m CT X SCWL m a t i n g s do n o t e x h i b i t a c a r r y - o v e r o f m a t e r n a l F^ g e n e r a t i o n ( T a b l e V I ) .  e f f e c t s from t h e  The p r o g e n y d e s c e n d a n t f r o m CT X NH  m a t i n g s c o n t i n u e t o h a v e a s i g n i f i c a n t l y h i g h e r CT i n c i d e n c e i n t h e NH cf X CT ? ? l i n e t h a n i n t h e CT cf X NH ? ? l i n e 1 4 . 9 8 3 , P =/.01 w i t h 1 d . f . ) .  ( c h i square =  T h i s d i f f e r e n c e between t h e  r e c i p r o c a l p a r e n t a l l i n e s i s not supported  i nthe backcross  where no d i f f e r e n c e s i n i n c i d e n c e o c c u r i n t h e b a c k c r o s s e s t o t h e NH l i n e o r t o t h e CT l i n e . carry-over of maternal CT X NH b r e e d  matings either  T h e r e f o r e , t h e r e may be a  e f f e c t s i n t o t h e F2 g e n e r a t i o n f r o m t h e  crosses althoughthe  evidence  i s inconclusive.  Table VI I n c i d e n c e o f CT i n t h e P-^, F-^, P2» a n d b a c k c r o s s m a t i n g s i n each o f t h e f o u r l i n e s formed i n t h e experiment.  Combined  data i n percent.  Line  CT 0* X SCWL  SCWL cf X CT ?.?  l SCWL  BC t o  BC t o  SCWL ? $  CT cf  CT  10.2  9.3  84.3  91.3  x  10.2  NH CT cf X NH  NH cf X CT ? ?  2.5  2.5  18.5  45.9  BC t o  BC t o  SCWL cf  CT g ?  21.4  57.7  38.7  2.8  20.6  90.3  BC t o  BC t o  NH cf  CT  8.7  91.3  BC t o CT cf.  BC NH  9.0  n o t made  17.5  32.5  91.8  91.3  91.3  22  Inbreeding T h e r e a r e two l e v e l s o f i n b r e e d i n g b e t w e e n i n t h e F2 g e n e r a t i o n .  progenies  M a t i n g s o f t h e F-^ b i r d s w h e r e v e r p o s s i b l e  were b e t w e e n h a l f s i b s w h i c h p r o d u c e d p r o g e n y w i t h P = .194. I n most s i r e f a m i l i e s , h o w e v e r , i t was n e c e s s a r y one  full  s i s t e r t o the s i r e i nthe breeding  s u f f i c i e n t number o f dams.  t o place at least  pen t o o b t a i n a  Progeny from f u l l  s i b matings had an  i n b r e e d i n g c o e f f i c i e n t o f .388. C o m p a r i s o n s b e t w e e n dam f a m i l i e s w i t h i n s i r e f a m i l i e s  the  showed t h a t i n h a l f ^ s i r e f a m i l i e s ( s i x o u t o f 1 2 ; t h e p r o g e n y w i t h t h e h i g h e r i n b r e e d i n g c o e f f i c i e n t were c o n s i s t e n t l y ^ h i g h e r i n CT i n c i d e n c e t h a n t h o s e w i t h t h e l o w e r  inbreeding  coefficient.  When t h e s i r e f a m i l i e s a r e c o m b i n e d f o r c o m p a r i s o n s b e t w e e n t h e f o u r c l a s s e s o f P^ m a t i n g s , t h e CT i n c i d e n c e i s h i g h e r i n t h e more i n b r e d p r o g e n y i n a l l l i n e s e x c e p t CT o*X NH CT i n c i d e n c e i n h a l f s i b s a n d f u l l in  s i b s i n F2  generation  percent. p = .194  P = .388  Weighted  CT cf X SCWL ¥?  83/214  38.8  64/108  59.2  SCWL cf X CT  ?$  72/183  39.3  16/29  55.2  .216  CT cf X NH  ??  63/290  21.7  31/165  18.8  .267  39/131  29.8  71/177  40.1  .291  CT X SCWL (combined)  155/397  39.0  80/137  58.4 **  .237  CT X NH (combined)  102/421  24.2  102/342  29.8  .277  NH cf X CT  ** The  **  .259  S i g n i f i c a n t a t 1% l e v e l .  d i f f e r e n c e between t h e l e v e l s o f i n c i d e n c e a s s o c i a t e d w i t h  t h e two d e g r e e s o f i n b r e e d i n g i n t h e CT o*X SCWL statistically  significant.  line i s  When t h e r e c i p r o c a l p a r e n t a l  a r e c o m b i n e d t h e CT i n c i d e n c e i s h i g h e r i n t h e f u l l  lines  sibs i n  23  both t h e s t r a i n c r o s s and the breed  cross.  t h e CT X SCWL l i n e s r e m a i n s s t a t i s t i c a l l y  The d i f f e r e n c e s i n significant.  I f t h e h i g h e r CT i n c i d e n c e i n t h e more h i g h l y i n b r e d F  2  zygotes  i s a phenomenon o f i n b r e e d i n g d e g e n e r a t i o n ,  w o u l d be e x p e c t e d these  zygotes.  that a lowered  then i t  h a t c h a b i l i t y w o u l d o c c u r among  T h i s d o e s n o t h a p p e n a s i s shown b e l o w .  H a t c h a b i l i t y i n h a l f s i b s and f u l l  sibs i nF  F = .194  generation.  2  P = .388  CT X SCWL  232/318  73.0$  100/147  68.0$  CT X NH  366/450  81.3$  306/374  81.8$  A s s o c i a t i o n w i t h Sex T h e r e was no a s s o c i a t i o n o f CT i n c i d e n c e w i t h s e x evident i n the data. although  S e x r a t i o s were n o r m a l i n c o m b i n e d  data  c o n s i d e r a b l e v a r i a t i o n e x i s t e d b e t w e e n dam f a m i l i e s . S e c o n d a r y s e x r a t i o s i n CT e x p e r i m e n t ,  a l l zygotes  combined. Generation P F The  Males  Sex r a t i o  451  458  49.61  1063  1003  51.45  1  2  Females  r e l a t i o n b e t w e e n CT i n c i d e n c e a n d s e x showed no d i f f e r e n c e s  between males and females. Incidence  o f CT i n :  Males  Females  F^  generation  13.6$  14.8$  F  generation  32.3$  30.2$  9  24  I n t h e d e a d embryos o f t h e  g e n e r a t i o n , 67.8$ o f t h e m a l e s  were C I a n d 60.0$ o f t h e f e m a l e s  were CT.  A t t e n weeks o f a g e ,  69 CT b i r d s h a d d i e d (34 m a l e s a n d 35 f e m a l e s ) . suggest  The d a t a  t h a t t h e e x p r e s s i o n o f CT a c t s e q u a l l y on m a l e s a n d  females. Discussion One  o f t h e most s t r i k i n g f e a t u r e s o f t h e d a t a i s t h e  t r e m e n d o u s v a r i a b i l i t y b e t w e e n t h e p h e n o t y p e s o f t h e dams a n d their offspring.  This v a r i a b i l i t y persists t o a considerable  e x t e n t even t o t h e l e v e l o f c o m p a r i s o n s between s i r e  families  (note p a r t i c u l a r i l y t h e v a r i a b i l i t y between s i r e f a m i l i e s i n Table I V ) .  The p e r f o r m a n c e o f t h e p a r e n t s g i v e s b u t a s l i g h t  i n d i c a t i o n o f what t o e x p e c t  i n the performance o f t h e o f f s p r i n g  w i t h r e l a t i o n t o t h e t r a n s m i s s i o n o f CT i n c i d e n c e ( f o r e x a m p l e , no  d e f i n i t i v e segregation r a t i o s ) .  CT  t r a i t provides strong evidence  This c h a r a c t e r i s t i c of the o f a complex mechanism o f  i n h e r i t a n c e , one t h a t i s n o t a m e n a b l e t o a s i m p l e  Mendelian  analysis. The  data  (Table V I ) i n d i c a t e that breed d i f f e r e n c e s  e x i s t a t l e a s t d u r i n g the f i r s t combination breeds. and  breed  two g e n e r a t i o n s a f t e r t h e  o f t h e CT genome w i t h t h a t o f t h e UBC SCWL a n d NH  The d i f f e r e n c e s b e t w e e n t h e CT i n c i d e n c e i n t h e s t r a i n c r o s s e s may i n p a r t be a t t r i b u t e d t o d i f f e r e n t  olds f o r the expression o f the character.  thresh-  The w i d e r c r o s s t o  t h e NH l i n e v e r y l i k e l y p r o v i d e s a g r e a t e r number o f h e t e r o z y g o u s l o c i i n t h e F^ g e n e r a t i o n .  The c o n s e q u e n c e o f t h i s  w o u l d be a t e n d e n c y t o w a r d  a h i g h e r t h r e s h o l d f o r CT e x p r e s s i o n  as w e l l a s t h e e x p r e s s i o n o f h y b r i d supported  situation  vigor..  This hypothesis i s  b y t h e r e l a t i v e l y l o w i n c i d e n c e o f CT i n t h e  g e n e r a t i o n o f p r o g e n y f r o m t h e r e c i p r o c a l CT X NH m a t i n g s 2 and 3). A l s o , h y b r i d v i g o r i s suggested  by a g r e a t e r h a t c h -  a b i l i t y i n t h e r e c i p r o c a l CT X NH P-^ g e n e r a t i o n t h e r e c i p r o c a l CT X SCWL P That t h e t r a i t  1  generation  (Pigs.  (91.7$) t h a n i n  (76.6$).  i s s t r o n g l y subject t o environmental  m o d i f i c a t i o n i s e v i n c e d by t h e e f f e c t s o f t h e m a t e r n a l  environ-  25  merit.  The m a t e r n a l  e f f e c t h a s i t s p r i m a r y i n f l u e n c e on p h e n o -  t y p i c e x p r e s s i o n and d o e s n o t a l t e r t h e z y g o t i c g e n o t y p i c p o t e n t i a l s f o r CT e x p r e s s i o n , f o r e x a m p l e , t h e b a c k c r o s s e s i n t h e CT X NH c r o s s e s .  I t reduces  so t h a t more z y g o t e s d e v e l o p  t h e t h r e s h o l d f o r CT  the defect than others w i t h  e q u i v a l e n t g e n o t y p e s w h i c h developenvironmentalc onditions.  expression  u n d e r more f a v o u r a b l e  I t i s probable  t h a t a l o w e r CT  ence w o u l d p r e v a i l i n t h e F-^ g e n e r a t i o n b a r r i n g m a t e r n a l Even i n t h e presence  of maternal  effects the  incideffects.  incidence i nthe  CT X NH c r o s s e s a p p r o a c h e s v e r y c l o s e l y t h e i n c i d e n c e e n c o u n t e r e d i n t h e NH p a r e n t a l s t r a i n .  I n f a c t , i n t h e CT o"X NH  c r o s s t h e i n c i d e n c e i n t h e P^ g e n e r a t i o n was 2.8$ ( T a b l e V I ) which i s almost matings (2.5$).  i d e n t i c a l w i t h t h e i n c i d e n c e i n t h e p u r e NH The e v i d e n c e  i n d i c a t e s t h a t t h e CT g e n e s c o n d -  i t i o n different thresholds f o r expression of the defect.  Thus  a t h r e s h o l d w h i c h w o u l d n o t b e e x c e e d e d i n c o n d i t i o n s o f optimum e n v i r o n m e n t may be t r i g g e r e d b y a s u b o p t i m a l m a t e r n a l  environment  o r o t h e r d e l e t e r i o u s environments as o c c u r r e d , f o r example, i n the temperature  experiments  i n " t h e UBC E x p e r i m e n t a l l i n e s .  (Data  unpublished). The  e f f e c t of temperature High  o n CT i n c i d e n c e , i n p e r c e n t .  temperature  Low  98.0°P  101v25°P Embryonic  temperature  At hatch  Embryonic  At hatch  Inbred  41.8  4.2  13.0  0.0  Non-inbred  34.3  0.0  7.5  0.0  Crossbred  33.3  0.0  0.0  0.0  The  effect of inbreeding i s apparently to increase  CT i n c i d e n c e i n d e p e n d e n t l y  of inbreeding degeneration.  i c a l a n a l y s i s of the data suggests  Statist-  that t h e assumption that  c h a n c e f l u c t u a t i o n i n gene f r e q u e n c i e s o f s p e c i f i c  CT  d e t e r m i n i n g genes ( g e n e t i c d r i f t ) a l o n e i s i n s u f f i c i e n t t o account  f o r t h e d i f f e r e n c e s between d i f f e r e n t l e v e l s o f inbreed-  26  ing.  The  e v i d e n c e on t h i s p o i n t  t h a t t h e CT  character  i s important i n that i t i n d i c a t e s  i s i n f l u e n c e d by h o m o z y g o s i t y p e r  Thus- i t i s d e m o n s t r a t e d t h a t t h e r e s u l t of inbreeding the r e s u l t of the i n f l u e n c e d by  expression  degeneration- but  same u n d e r l y i n g  o f CT  se.  i s not  the  r a t h e r - b o t h phenomena  cause.  That i s , b o t h  i n c r e a s i n g h o m o z y g o s i t y i n an o r g a n i s m  are  that  d e p e n d s u p o n a h i g h l y h e t e r o z y g o u s c o n d i t i o n f o r optimum The generations The  c o m b i n e d d a t a ( P i g s . 2 and  i n d i c a t e t h e CT  g e n e s t o be  3) f o r t h e F^ incompletely  genes.  Further  f o u n d w i t h i n t h e F2 parental crosses t y p e s (see  the be  when a l l m a t i n g s f r o m t h e  four  c o m b i n e d on t h e b a s i s o f t h e p a r e n t a l  ¥  ±  ST  U  ST  142/489  29.0$  1  CT  266/778  34.2$  x  CT  106/264  40.2$  1  ST X P  F-L CT  X F  There i s a rough correspondence between the phenotype of p a r e n t s and  t h e CT  incidence  i n the  o f f s p r i n g but  the  the  high  i n t h e ST X ST m a t i n g s r e l a t i v e t o t h a t i n t h e CT  matings i n d i c a t e s v a r i a b l e penetrance. of v a r i a b l e penetrance occurred 1949  pheno-  below). . F_  incidence  P2  dominant.  e v i d e n c e o f t h i s a c t i o n may  generation  are  fitness.  and  maternal e f f e c t i n d i c a t e s a v a r i a b l e penetrance i n  a c t i o n o f CT  are  when t h e p r o g e n y o f ST  s e l e c t i o n l i n e had the g e n e r a t i o n  A more s t r i k i n g  i n t h e C a l i f o r n i a CT  representatives  a s h i g h an  incidence  f r o m w h i c h t h e ST  o f CT  CT  example  line  o f t h e CT  X  in  positive  as o c c u r e d i n  p a r e n t a l b i r d s came  (Hicks,  1953). The incidence  m a t e r n a l and  o f CT  inbreeding  effects indicate  the  i s a f f e c t e d by v a r y i n g t h r e s h o l d l e v e l s c a u s e d  by g e n e - e n v i r o n m e n t i n t e r a c t i o n s and  v a r y i n g d e g r e e s o f homozy-  gosity.  o f CT  The  ( F i g s . 2 and provides  fact that  the i n c i d e n c e  3) l i e s b e t w e e n t h e i n c i d e n c e  i n t h e F-j_  i n the p a r e n t a l  evidence of a p a r t i a l l y q u a n t i t a t i v e genetic  a t i o n o f t h e CT  trait.  generation lines  determin-  I f i t i s assumed t h a t t h e h i g h e r  level  27  o f CT i n c i d e n c e i n t h e F g g e n e r a t i o n i s p r i m a r i l y due t o a l o w e r t h r e s h o l d r e s u l t i n g from s e g r a t i o n and t h e p r o d u c t i o n o f a g r e a t e r number o f homozygous l o c i , t h e n t h e q u a n t i t a t i v e characteristic of the t r a i t evidence  i s found  becomes more e v i d e n t .  Further  i n t h e b a c k c r o s s m a t i n g s t o t h e ST l i n e s  where t h e p r o g e n y e x h i b i t a CT i n c i d e n c e i n t e r m e d i a t e t o t h a t in the parental lines.  However, t h e CT i n c i d e n c e a p p r o a c h e s  more c l o s e l y t h a t i n t h e p a r e n t a l ST l i n e s . The  b e s t h y p o t h e s i s w h i c h f i t s t h e d a t a i s one w h i c h  assumes a number o f g e n e s w h i c h c o n d i t i o n t h e d e v e l o p m e n t o f t h e extremities of the posterior limb.  T h e s e g e n e s , w h i c h may be  r e c e s s i v e , a r e p r e s e n t i n most i f n o t a l l p o p u l a t i o n s o f c h i c k e n s and t h e y a r e amenable t o s e l e c t i o n w i t h i n b r e e d i n g . Thus, i n each g e n e r a t i o n o f a n o n - s e l e c t e d p o p u l a t i o n t h e r e w i l l be a c e r t a i n number o f i n d i v i d u a l s w h i c h h a v e one o r more loci  i n t h e homozygous s t a t e a n d w h i c h w i l l e x h i b i t t h e t r a i t  phenotypically.  S e l e c t i o n f o r increased frequency  leads t o the accumulation l o c i and c o n s e q u e n t l y  o f t h e genes  o f a g r e a t e r p r o p o r t i o n o f homozygous  t o a h i g h e r i n c i d e n c e and e x p r e s s i o n o f  the phenotype. D i f f e r e n t p o p u l a t i o n s may c a r r y d i f f e r e n t t i o n s o f g e n e s a t t h e CT l o c i .  T h i s would account  constellaf o r the  v a r i a b l e i n c i d e n c e s i n t h e F^ g e n e r a t i o n s w h i c h were d e r i v e d from d i f f e r e n t s t r a i n and breed  c r o s s e s w i t h t h e CT l i n e .  When t h e s e F^ b i r d s a r e b a c k c r o s s e d  t o a parental line  l o w i n c i d e n c e , i t w o u l d be e x p e c t e d  t h a t t h e i n c i d e n c e w o u l d be  approximately  with  equal t o t h a t i n t h e p a r e n t a l l i n e because t h e  b a c k c r o s s p r o g e n y w o u l d be homozygous o n l y a t t h o s e l o c i  which  r e c e i v e d genes f r o m t h e l o w i n c i d e n c e P^ l i n e . E s s e n t i a l l y t h e same l i n e o f r e a s o n i n g c a n be a p p l i e d to  an e x p l a n a t i o n o f t h e evidence  line.  i nthe backcross  F^ b i r d s c a r r y a l a r g e s a m p l i n g  t o t h e CT  o f CT g e n e s , a p r o p o r -  t i o n o f w h i c h a r e i n t h e homozygous c o n d i t i o n .  When r e i n t r o d -  u c e d i n t o t h e CT genome, a l l o f t h o s e l o c i w h i c h were homozygous w i l l p r o b a b l y r e m a i n s o a n d a t l e a s t one h a l f o f t h e  28  l o c i which were heterozygous in the F-^ birds w i l l become homozygous in the backcross zygotes. The presence of closely linked gene constellations with predominately CT genes which very likely occur in the CT parental line, w i l l tend to increase the proportion of l o c i which become homozygous over that estimated above. Thus a considerably elevated incidence of CT will occur in such backcrosses. The original hypothesis can be extended to account for more aspects of the data by certain assumptions relative to the quantitative aspects of the number of homozygous l o c i . During the course of the original selection for the increased incidence of CT (Hicks, 1953), the pattern of increases in the early generations followed a roughly quantitative response to selection (1945-46). The intermediate generations did not (1947-48). On the basis of the selection differential the increases due to selection in the intermediate generations were considerably greater than expected. Thus, as the number of homozygous l o c i is increased, the phenotypic expression of CT increases proportionately until a point is reached where the addition of s t i l l more homozygous l o c i produces a geometric increase in incidence. The backcross incidence in the P-^ X CT parental lines is amenable to such an explanation.  29 EXPRESSIVITY The  CT c h a r a c t e r was c l a s s i f i e d i n t o f o u r g r a d e s  d e p e n d i n g u p o n t h e d e g r e e o f i t s e x p r e s s i o n , a f t e r t h e method o f Hicks  (1953).  T h i s c l a s s i f i c a t i o n i s g i v e n below: Classification  Straight  toes  S l i g h t crooked  toes  Moderate crooked Severe crooked  toes toes  Grade  Curvature  1  none  2  0-45  3  45-90°  4  over 90  u  c  B i l a t e r a l a s y m m e t r y , w h i c h i s v e r y common i n t h i s m a t e r i a l , was i g n o r e d when a s s i g n i n g a c h i c k t o a p a r t i c u l a r g r a d e i n t h a t t h e f o o t w i t h t h e g r e a t e s t e x p r e s s i o n o f CT was t h e c r i t e r i o n f o r classification. The  e x p r e s s i v i t y c l a s s i f i c a t i o n s were made a t t h e t i m e  the c h i c k s hatched.  The m a t i n g s i n t h e F ^ g e n e r a t i o n  (Table V I I ) ,  w i t h t h e e x c e p t i o n o f t h e CT X CT m a t i n g , show t h a t t h e m a j o r i t y of b i r d s f a l l w i t h i n the l o w grade phenotypes. m a t i n g s where m a t e r n a l  e f f e c t s a r e present  phenotypes a r e present  (see  CT  cf X NH  NH c f X CT On  No. o f c h i c k s i n g r a d e  cf X SCWL ft  SCWL cf X CT  t h a t t h e s e v e r e CT  below).  Mating  CT  I t i sonly i n the  n  1  :  2  5  3  :  4  131  :  8  •  4  :  0  11  ;  1  :  4  2  :  0  :  0  1  :  0  :  1  19  :  ft  161  :  ft  32  :  t h e o t h e r h a n d , i n t h e CT X CT m a t i n g t h e g r e a t e s t p r o p o r t i o n  o f CT b i r d s h a v e s e v e r e  phenotypes (85.7$).  I n t h e m a t i n g s where  t h e CT l i n e b i r d s were c r o s s e d w i t h t h e UBC SCWL a n d NH l i n e b i r d s a n d t h e m a t i n g s w i t h i n t h e UBC l i n e s , t h e r e i s e s s e n t i a l l y  Table 711 E x p r e s s i v i t y o f CT t r a i t .  C h i c k s grouped a c c o r d i n g t o grade  of e x p r e s s i o n (see t e x t ) .  Includes a l l hatched c h i c k s .  Mating category  P,  T o t a l No. chicks  No. c h i c k s i n g r a d e 1 : 2 : 3 : 4  $ c h i c k s i n grade 1 : 2 : 3  4  :  combined reciprocal 28  2  !:  1  ::  1  : 24  CT X SCWL  178  150  .  19  .  5  :  CT X NH  197  193  3  .  0  :  SCWL X SCWL  460  459  552  354  58  SCWL  307  297  1  CT  130  22  18  714  563  54  NH  342  328  CT  99  8  CT X CT  7.1  :  3.6  .  3.6  : 85.7  4  84.3  :  10.7  :  2.8  :  2.2  1  98.0  :  1.5  ;  0.0  :  0.5  99.8  0.2  0.0  0.0  SCWL X NH NH X NH F,  combined recprocal CT X SCWL F F F  X F  x  1  X P  1  X P  x  1  x  : 44  : 96  64.1  .  10.5  :  8.0  :  17.4  :  :  6  96.7  .  0.3  :  1.0  .:  2.0  : 17  : 73  16.9  ..  13.8  : 1 3 . 1 ':  56.2  : 22  : 75  78.8  :  7.6  :  3.1  i  10.5  :  :  3  95.9  2.6  :  0.6  :  0.9  : 46  8.1  : 24.2 ..  46.5  3  !T X NH p  ¥ F  l  .  x  X P  1  l  p  x  p  l  l x  .  9 :  21  2  : 24  !  21.2  31  a d e c r e a s i n g p r o p o r t i o n of c h i c k s w i t h h i g h grade phenotypes. I n t h e ]?2 g e n e r a t i o n t h e r e i s an i n c r e a s e i n t h e p r o p o r t i o n o f b i r d s w i t h CT p h e n o t y p e s . r e c i p r o c a l CT X SCWL p a r e n t a l m a t i n g s , e x h i b i t e d CT  15.7$  o f t h e F^  p h e n o t y p e s compared w i t h 35.9$ i n t h e P  w h i l e the weighted 1.78  I n the progeny from  (Pig. 4).  chicks generation  2  a v e r a g e e x p r e s s i v i t y i n c r e a s e d f r o m 1.22  A s i m i l a r increase i s present  s p r i n g f r o m t h e r e c i p r o c a l CT X NH  the  to  i n matings of  off-  p a r e n t a l matings ( P i g . 5).  I t i s i n t e r e s t i n g t o n o t e t h a t the s h i f t t o w a r d  greater  express-  i v i t y i s a p p a r e n t l y not p r o g r e s s i v e i n t h i s experimental m a t e r i a l . D u r i n g t h e d e v e l o p m e n t o f t h e CT the t r a i t  line i n California,  showed a p r o g r e s s i v e i n c r e a s e i n CT  incidence  w i t h a p r o g r e s s i v e s h i f t i n g i n e x p r e s s i v i t y from low phenotypes, through phenotypes.  The  coupled  grade  t h e i n t e r m e d i a t e g r a d e s t o the s e v e r e  l i n e s f r o m t h e s t r a i n and b r e e d  CT  c r o s s e s show  i n c r e a s e d numbers o f b i r d s i n t h e t h r e e g r a d e s r e p r e s e n t i n g phenotypic  e x p r e s s i o n of the t r a i t .  I n s t e a d of t h e  greatest  p r o p o r t i o n a l i n c r e a s e o c c u r r i n g i n the intermediate phenotypes, it  o c c u r r e d i n t h e s e v e r e CT  phenotype.  T h i s d i f f e r e n c e i n t h e a c t i o n o f t h e CT r e l a t i o n t o e x p r e s s i v i t y may  be  gene c o m p l e x i n  explained i f i t i s a polygenic  s y s t e m composed o f g r o u p s o f gene b l o c k s o r a g g r e g a t e s . t h e d e v e l o p m e n t o f t h e CT CT  line,  'During  a g r a d u a l change o c c u r r e d a s  the  d e t e r m i n i n g g e n e s were i n c r e a s e d so t h a t a h i g h l y h e t e r o -  g e n o u s s y s t e m became a t l e a s t r e l a t i v e l y homogeneous f o r t h e s e genes.  A number o f g e n e r a t i o n s w e r e r e q u i r e d f o r t h e  o f s e g r e g a t i o n , gene r e c o m b i n a t i o n , w i t h i n i n d i v i d u a l s enough CT transmitting ability.  and  processes  s e l e c t i o n , to accumulate  g e n e s f o r them t o h a v e a  high  A f u r t h e r d e l a y i n o b t a i n i n g CT  genotypes  w i t h a h i g h t r a n s m i t t i n g p o t e n t i a l would occur i f c r o s s i n g over among c l o s e l y l i n k e d gene a g g r e g a t e s t h e c a s e , an i m p o r t a n t s e l e c t i o n w o u l d be gene a g g r e g a t e s CT  was  necessary.  e f f e c t of c r o s s i n g o v e r u n d e r  the g r a d u a l accumulation  o f CT  I f t h i s were continued  within closely  linked  d e t e r m i n i n g genes w h i c h would i n c r e a s e t h e i r  p o t e n t i a l r e l a t i v e to unselected  birds.  32  UJ  <  oc-  QC  UJ  > <  UJ  UJ £  l - O C -  aoo  p,  x  SCWL F i g u r e 4.  Note:  BC  x  SCWL  BC  CT  X  X  C T  CT  E x p r e s s i v i t y i n r e c i p r o c a l CT X SCWL matings. Recorded as weighted average; see t e x t . V a l u e 1.00 r e p r e s e n t s ST p h e n o t y p e . V a l u e 4.00 r e p r e s e n t s s e v e r e CT p h e n o t y p e .  33  UJ  300-  _ LU  O  200-  UJ  O.OO  F i g u r e 5.  Note:  E x p r e s s i v i t y i n r e c i p r o c a l CT X NH matings. Recorded as weighted average; see t e x t . V a l u e 1.00 V a l u e 4.00  r e p r e s e n t s ST p h e n o t y p e . r e p r e s e n t s s e v e r e CT p h e n o t y p e .  W i t h t h e i n t r o d u c t i o n o f t h e CT genome w i t h a l a r g e number o f CT w i t h one  h a v i n g few  CT  l i n e t o UBC  d e t e r m i n i n g l o c i was  determining l o c i .  I n t h e F^  t h e h e t e r o z y g o s i t y r e s u l t i n g from s t r a i n and p r o d u c e s a h i g h t h r e s h o l d f o r CT  s e g r e g a t i o n and  i n the  recombination  The expressivity  The  ( F i g s . 4 and  phenotypes although the  a l l the grades.  g r a d e 1 c h i c k s X 1 + No.  T h i s average  grade 2 c h i c k s X 2 +  grade 4 c h i c k s X  4  of c h i c k s i f a l l the  i f a l l t h e c h i c k s were I n  the  class. The  had  the  formula:  c h i c k s were s t r a i g h t t o e d , t o 4.00  c l a s s i f i c a t i o n of the e x p r e s s i v i t y d a t a i n F i g s . 4  5 i s t h e same a s t h a t o f t h e CT  I t was  gene  generation.  Thus t h e w e i g h t e d a v e r a g e v a l u e c o u l d r a n g e f r o m 1.00  and  by  5) t h a t p e r m i t t e d c o m p a r i s o n s u s i n g  T o t a l No.  CT  CT  d a t a were c o m b i n e d i n t o a w e i g h t e d a v e r a g e o f  + No.  severe  the  threshold f o r  r e m a i n t h e same a s i n t h e F-^  obtained u s i n g the No.  crosses  o f homozygous g e n o t y p e s w h i c h  only a single value to represent was  generation,  g e n e r a t i o n by i n b r e e d i n g and  p e r m i t s a g r e a t e r i n c i d e n c e of CT frequencies probably  breed  a  combined  e x p r e s s i o n and m i n i m i z e s  number o f p h e n o t y p e s s h o w i n g t h e t r a i t . expression i s lowered  stock,  i n c i d e n c e i n F i g s . 2 and  3.  n o t p o s s i b l e t o c l a s s i f y t h e d e g r e e t o w h i c h d e a d embryos  developed  CT  b e c a u s e t h e e x p r e s s i v i t y o f t h e CT  w o u l d v a r y f r o m one  character  developmental stage t o another.  Omission of  t h e dead embryo component f r o m t h e e x p r e s s i v i t y d a t a g e n e r a l l y depress the v a l u e s , p a r t i c u l a r l y i n those  would categories  where e x p r e s s i v i t y i s l o w , when t h e y a r e compared w i t h F i g s . 2 and  3 ( r e c a l l t h a t e m b r y o n i c dead and  combined t o g i v e these v a l u e s ) .  at hatch incidence  I t w o u l d n o t be  expected  t h e o m i s s i o n o f t h i s component w o u l d s e r i o u s l y a f f e c t  are that  the  r e l a t i o n s h i p s b e t w e e n t h e m a t i n g c a t e g o r i e s w i t h i n F i g s . 4 and T h e s e r e l a t i o n s h i p s a r e t h e same a s In  those  i n t h e i n c i d e n c e of  general, there i s a very close c o r r e l a t i o n of e x p r e s s i v i t y  5 CT  35  with incidence.  A s t h e CT i n c i d e n c e i n c r e a s e s , t h e e x p r e s s i v i t y  s h i f t s increasing the proportion o f b i r d s e x h i b i t i n g the  higher  grade phenotype. ASSOCIATION OF CT WITH FITNESS F i t n e s s i s a very g e n e r a l term u s u a l l y a s s o c i a t e d f a c t o r s such as f e r t i l i t y , ability.  with  f a m i l y s i z e , v i g o r , and s u r v i v a l  I n c h i c k e n s , h a t c h a b i l i t y o f f e r t i l e eggs i s one o f t h e  most u s e f u l s i n g l e m e a s u r e s o f f i t n e s s . Co-adaptation A l l t h e m a j o r m a t i n g c a t e g o r i e s were t e s t e d t o d e t e r m i n e t h e r e l a t i o n b e t w e e n t h e CT c h a r a c t e r a n d f i t n e s s . by c o m p a r i n g t h e d i f f e r e n t i a l l i v e a b i l i t y  T h i s was done  o f CT a n d ST z y g o t e s o n  the b a s i s o f t h e p r o p o r t i o n h a t c h i n g a g a i n s t t h e p r o p o r t i o n  dying  during incubation.  with  I f ST z y g o t e s  f i t n e s s , a n d CT z y g o t e s c a n be e v a l u a t e d more CT z y g o t e s  are considered  co-adapted  a r e compared w i t h them, t h e n t h e CT  regarding co-adaptedness. d i e t h a n ST z y g o t e s ,  t h e y were n o t c o - a d a p t e d .  zygotes  I fproportionately  t h e evidence would i n d i c a t e  I f t h e p r o p o r t i o n s a r e t h e same, no  r e l a t i o n s h i p i s e v i d e n t u n l e s s t h e y may be t e r m e d e q u a l l y c o adapted.  I f p r o p o r t i o n a t e l y f e w e r CT z y g o t e s  considered  d i e , t h e y w o u l d be  co-adapted. I n t h e p a r e n t a l m a t i n g s ( T a b l e V I I I ) t h e r e was e v i d e n c e  of co-adaptation CT  X CT mating;-,  o f CT i n c i d e n c e w i t h e n b r y o n i c This c o n d i t i o n arose  liveability  i n the  i n t h e C a l i f o r n i a CT l i n e  d u r i n g t h e s e l e c t i o n experiment ( H i c k s , 1953).  On t h e o t h e r  h a n d , i n t h e SCWL X SCWL m a t i n g t h e CT i n c i d e n c e i n t h e e m b r y o n i c dead i s s i g n i f i c a n t l y h i g h e r 1 d.f.)  ( c h i s q u a r e = 2 9 . 5 3 1 , P = Z «01 w i t h  than that i nt h e hatched c h i c k s which i n d i c a t e s that i n  t h i s c a s e i i e r e i s no c o - a d a p t a t i o n liveability.  o f CT d e f e c t w i t h  I n t h e NH X NH m a t i n g s a l l t h e CT z y g o t e s  d u r i n g i n c u b a t i o n b u t t h e number o f CT z y g o t e s i nthis  mating.  died  was t o o s m a l l t o  obtain s t a t i s t i c a l v e r i f i c a t i o n that co-adaptation present  embryonic  was n o t  36  The r e c i p r o c a l p a r e n t a l s t r a i n a n d b r e e d c r o s s e s t o t h e C a l i f o r n i a CT l i n e  ( T a b l e V I I I ) i n d i c a t e no c o - a d a p t a t i o n  occurred  b e t w e e n e m b r y o n i c l i v e a b i l i t y a n d t h e CT c h a r a c t e r .  I n each  c r o s s t h e i n c i d e n c e o f CT i n t h e e m b r y o n i c d e a d was s i g n i f i c a n t l y higher than i n the hatched c h i c k s . I n o v e r h a l f Of t h e s i r e f a m i l i e s f r o m F-^ m a t i n g s out  o f 1 6 ) a s i g n i f i c a n t l y h i g h e r CT i n c i d e n c e among  dead o c c u r r e d . and  F  1  T h r e e .of t h e r e m a i n i n g  nine  embryonic  m a t i n g s , (F-^ ST cf X F-^ ST  ST cf X F, CT ? ? f r o m CT cf X SCWL %%, a n d J? S I cf X F ^ CT ±  f r o m CT cf X NH ¥ ? ) , h a d a n e q u a l o r low@r i n c i d e n c e o f CT i n t h e embryonic dead.  These m a t i n g s suggest t h a t c o - a d a p t a t i o n  i n c i d e n c e w i t h embryonic l i v e a b i l i t y h a s o c c u r r e d .  o f CT  I f , however,  t h e F ^ m a t i n g s a r e c o m b i n e d so t h a t a l l f o u r c l a s s e s o f m a t i n g s from each p a r e n t a l m a t i n g have a s i n g l e v a l u e , t h e n t h e F^ X F^ m a t i n g s o f CT cf X SCWL  p a r e n t a l m a t i n g have a s i g n i f i c a n t l y  h i g h e r CT i n c i d e n c e i n t h e e m b r y o n i c dead ( c h i s q u a r e = 1 7 . 2 5 0 , P = / .01 w i t h 1 d . f . ) t h a n i n t h e h a t c h e d c h i c k s . The b a c k c r o s s  m a t i n g s t o t h e SCWL a n d NH l i n e s  g i v e no i n d i c a t i o n o f c o - a d a p t a t i o n  occurring.  (Table IX)  The e m b r y o n i c d e a d  have a c o n s i s t e n t l y h i g h e r i n c i d e n c e t h a t i s s t a t i s t i c a l l y icant.  This i scontrasted i n t h e backcross  m a t i n g s t o t h e CT l i n e  where t h e e m b r y o n i c dead e i t h e r do n o t h a v e s i g n i f i c a n t l y i n c i d e n c e o f CT o r h a v e a l o w e r  signifhigher  incidence.  I t seems l i k e l y t h a t t h e CT t r a i t number o f m o d i f i e r s a f f e c t i n g f i t n e s s .  i s associated with a  I n normal p o u l t r y f l o c k s  t h e s e m o d i f i e r s a r e p o s i t i v e l y a s s o c i a t e d w i t h ST p h e n o t y p e s a n d n e g a t i v e l y a s s o c i a t e d w i t h CT p h e n o t y p e s . m o r t a l i t y o f CT z y g o t e s  i n these  A high  embryonic  flocks i sindicated i n the  p a r e n t a l m a t i n g s o f t h e p u r e UBC l i n e s  (Table V I I I ) .  In the  C a l i f o r n i a CT l i n e , f i x a t i o n o f t h e t r a i t h a s r e s u l t e d i n a r e o r g a n i s a t i o n o f f i t n e s s m o d i f i e r s so t h a t t h e y became associated with fitness.  positively  P o s i t i v e a s s o c i a t i o n became p o s s i b l e  w i t h s e g r e g a t i o n and r e c o m b i n a t i o n  r e c u r r i n g o v e r a number o f  generations. When t h e CT l i n e was c o m b i n e d w i t h t h e SCWL a n d NH l i n e s ,  37 Table  VIII  C o m p a r i s o n o f CT i n c i d e n c e i n e m b r y o n i c d e a d ( 1 3 - 21 d a y s ) w i t h hatched  c h i c k s f o r evidence  with fitness.'  o f t h e CT c h a r a c t e r  a n d P2 g e n e r a t i o n s .  P, m a t i n g category  F., m a t i n g category  Sire  Sire  Dams  of co-adaptation  Dams  Embryonic dead No. CT/ total  io  Hatched c h i c k s No. CT/ total 26/28  92.8  CT X CT  16/18  88.9  SCWL X SCWL  17/73  23.3  1/121  0.8**  NH X NH  2/10  20.0  0/70  0.0  21/35  60.0  12/143  SCWL X CT  14/17  82.3  16/35  CT X NH  3/13  23.1  2/163  1.2**  NH X CT  5/6  83.3  2/34  5.9**  ST X ST  2/15  13.3  32/78  41.0  ST X CT  1/3  33.3  19/51  37.2  CT X ST  30/37  81.8  20/65  30.8**  CT X CT  15/16  93.8  28/55  50.9**  ST X ST  15/35  42.9  18/79  22.8*  ST X CT  7/15  46.7  12/69  17.4*  CT X ST  18/22  81.8  37/76  48.7*  CT X CT  13/18  72.2  32/79  40.5*  CT X SCWL  CT X SCWL  SCWL X CT  8.4** 45.7*  38  Table V I I I - continued P, m a t i n g categorySire  Dams  CT X NH  NH X CT  * **  mating  E m b r y o n i c dead  categorySire  Dams  No. CT/ total  Hatched  chicks  %  No. CT/ total  ST X ST  4/10  40.0  18/111  16.2  ST X CT  2/8  25.0  19/78  24.4  CT X ST  31/45  68.9  9/114  7.9**  CT X CT  6/17  35.3  5/72  6.9**  ST X ST  13/23  56.5  40/138  29.0**  ST X CT  1/2  50.0  3/35  8.6  CT X ST  6/8  75.0  51/139  36.7  CT X CT  1/1  6/26  23.1  S i g n i f i c a n t a t t h e 5$ l e v e l . S i g n i f i c a n t a t t h e 1% l e v e l .  100  39 Table I X Comparison  o f CT i n c i d e n c e i n e m b r y o n i c  d e a d ( 1 3 - 21 d a y s )  w i t h h a t c h e d c h i c k s f o r e v i d e n c e o f c o - a d a p t a t i o n o f t h e CT character with fitness.  Backcross  P, m a t i n g category  Backcross mating category  Sire  Sires  Dams  CT X SCWL  SCWL X CT CT X NH  NH X CT  **  F  l  P  l  P  l  F  l  P  l  P  l  F  l  Significant  X P CT X  F  1  matings.  CT X  F  l  NH X  F  l  X  P  x  l  chicks  No. CT/ total  SCWL  14/20  70.0  4/173  21/23  91.3  108/130  33/48  68.8  6/134  4.5**  10/20  50.0  10/203  4.9**  23/25  92.0  70/78  9/16  56.2  9/10  90.0  SCWL X ¥  ±  NH  C  T  a t t h e 1$ l e v e l .  $  Hatched  Dams  l  X P  Dead embryos  No. CT/ total  4/139 21/21  %  2.3** 83.1  89.7 2.9** 100.0  40  t h e z y g o t e s from, e a c h o f t h e s e c r o s s e s showed a n e g a t i v e iation with fitness.  assoc-  A p p a r e n t l y , i n t h e P^ g e n e r a t i o n , a  h a p l o i d s e t o f chromosomes f r o m t h e CT l i n e d o e s n o t c o n t a i n s u f f i c i e n t m o d i f i e r s t o m a i n t a i n a p o s i t i v e a s s o c i a t i o n o f CT with fitness.  However, s e g r e g a t i o n a n d r e c o m b i n a t i o n  matings should enable to  i n t h e P^  t h e s y s t e m o f m o d i f i e r s f r o m t h e CT l i n e  be l a r g e l y r e c o v e r e d  i n some o f t h e f a m i l i e s .  i n d i c a t e d i n t h e p r o g e n y o f t h r e e P^ s i r e  This i s possibly  families.  The b a c k c r o s s m a t i n g s p r o v i d e f u r t h e r i n f o r m a t i o n a b o u t these m o d i f i e r systems.  Whereas i n t h e P^ m a t i n g s ,  on t h e average,  h a l f o f t h e chromosomes i n e a c h o f f s p r i n g came f r o m one o f t h e p a r e n t a l l i n e s , i n t h e backcross matings,  on t h e a v e r a g e ,  three  f o u r t h s o f t h e chromsomes i n t h e p r o g e n y w i l l come f r o m t h e parental line.  C l e a r l y , i nthese matings a l a r g e part o f t h e  m o d i f i e r s y s t e m i n t h e p a r e n t a l l i n e , w i l l be r e c o v e r e d backcross  progeny.  Therefore  i t w o u l d be e x p e c t e d  i n the  t h a t t h e progeny  f r o m t h e s e m a t i n g s w o u l d e x h i b i t t h e same a s s o c i a t i o n o f CT w i t h f i t n e s s as t h a t i n t h e p a r e n t a l l i n e used i n t h e backcross; a s s o c i a t i o n s o f CT w i t h f i t n e s s p r e s e n t e d this  i nTable  The  IX support  hypothesis.  C o r r e l a t i o n o f CT I n c i d e n c e w i t h H a t c h a b i l i t y A comparison o f t h e h a t c h a b i l i t y o f t h e f e r t i l e f r o m t h e p a r e n t a l m a t i n g s w i t h t h a t i n t h e UBC p r o d u c t i o n  eggs flocks  shows t h a t i t was much l o w e r i n t h e e x p e r i m e n t a l m a t e r i a l . Hatchability i n :  UBC p r o d u c t i o n flock, 1 9 5 4 .  ($)  7 6 . 2  SCWL X SCWL NH X NH These v a l u e s suggest  8 6 . 8  =  . 5 5 ) CT  experiment,  P-L m a t i n g s , 1 9 5 5 . 5 8 . 7 7 6 . 3  t h a t s t r e s s c o n d i t i o n s were p r e s e n t i n t h e  experimental m a t e r i a l . (P  CT  X CT m a t i n g  The h a t c h a b i l i t i e s i n t h e h i g h l y i n b r e d ( 5 0 . 9 $ )  c r o s s e s NH X SCWL ( 8 5 . 8 $ ) ,  a n d t h e r e c i p r o c a l UBC b r e e d  indicate that the stress factors  could  41  be a c t i n g e q u a l l y on a l l t h e  experimental m a t e r i a l .  s h o u l d be s a f e t o assume t h a t  uniform external  a c t i n g on a l l t h e The inverse The  between m a t i n g s  which w i l l  tend to  differences  due t o  exaggerate d i f f e r e n c e s  analysis  maternal  a n d by h y b r i d  t e n d to  reduce  CT i n c i d e n c e . were s u b j e c t e d  to  a  o f dam f a m i l y h a t c h a b i l i t y on CT i n c i d e n c e .  T h i s was made on a l l F total  from e a c h o f t h e  z y g o t e s c o m b i n e d , and on t h e  2  r e c i p r o c a l P^ l i n e s  (see  combined  below).  Regression coefficient  "t"  d.f.  -0.6238  4.53**  62  -0.5449  1.90  16  SCWL cf X CT ? ?  -0.9080  3.93**  16  CT cf X NH  -0.5050  1.26  14  NH cf X CT ? ?  -0.5749  1.73  10  Combined t o t a l P zygotes 2  CT cf X SCWL ? ?  **  Significant  A significant combined  data.  significant  regression.  ative the  CT i n c i d e n c e The  CT X NH l i n e s regression  at  1$ l e v e l .  negative regression  However,  relationship  o n l y one l i n e  T h e r e was,  between the  increases,  there  however,  is  9  the  statistically neg-  w h i c h means t h a t  as  hatchability.  both p a r e n t a l s t r a i n s  zygotes.  in  a consistently  a decrease i n  (Table X I ) provide evidence of the F  was p r e s e n t  showed a  two v a r i a b l e s  backcross matings t o  analysis  an  of CT.  a r e c o m p l i c a t e d by t h e  D a t a from t h e P 2 g e n e r a t i o n regression  were  ( T a b l e X) i n d i c a t e  v i g o r i n s t r a i n and b r e e d c r o s s e s w h i c h w i l l  it  development.  r e l a t i o n s h i p between h a t c h a b i l i t y and i n c i d e n c e  relationships  effect  conditions  experimental m a t e r i a l during  h a t c h a b i l i t y o f F-^ z y g o t e s  Hence  supporting  in  the  the  U s i n g c h i square  analysis,  Table X H a t c h a b i l i t y and l i v e a b i l i t y with  fitness.  and ¥  0  P, m a t i n g category  P, m a t i n g category  Sire  Sire  Dams  Dams  (10 weeks) t o show a s s o c i a t i o n  generations.  CT  incidence^ No. CT/ total 42/46  CT X CT SCWL X NH  o f CT gene c o m p l e x  * 91.3  Hatchability  Liveability  No. c h i c k s / t o t a l embryos  No. b i r d s / t o t a l chicks  .  *  $  28/55  50.9  17/23  73.9  3/295  1.0  266/310  85.8  218/231  94.4  33/170  19.4  143/188  76.1  119/128  93.0  30/52  57.7  35/65  53.8  22/28  78.6  2.8  183/199  92.0  136/139  97.8  NH X SCWL' CT X SCWL. SCWL X CT CT X NH  5/178  NH X CT  7/40  17.5  19/31  61.3  24/27  88.9  ST X ST  34/93  36.6  78/98  79.6  73/78  93.6  ST X CT  20/54  37.0  53/56  94.6  49/51  96.1  CT X ST  50/102  49.0  65/104  62.5  64/65  98.5  CT X CT  43/71  60.6  57/82  69.5  50/55  90.9  CT X SCWL  ro  Table X - continued p.. m a t i n g category  P-, m a t i n g category  Sire  Sire  Dams  SCWI X CT  CT X NH  NH X CT  Dams  No. CT/ total  Hatchability  *  No. total  chicks/ embryos  Liveability  7°  No. b i r d s / total chicks  ST X ST  33/114  29.0  79/128  61.7  65/79  82.3  ST X CT  19/84  22.6  69/88  78.4  64/69  92.8  CT X ST  55/98  56.1  79/103  76.7  62/76  81.6  CT X CT  45/97 .  46.4  81/103  78.6  74/79  93.7  ST X ST  22/121  18.2  117/129  90.7  106/111  95.5  ST X CT  21/86  24.4  ' 81/95  85.3  76/78  97.4  CT X ST  40/159  25.2  115/175  65.7  100/114  87.7  CT X CT  11/89  12.4  74/101  73.3  67/72  93.1  ST.'X-ST  53/161  32.9  140/167  83.8  133/138  96.4  ST X CT  4/37  10.8  36/38  94.7  33/35  94.3  CT X ST  57/147  38.8  145/157  92.4  137/139  98.6  CT X CT  7/27  25.9  29/31  93.6  - 26/26  299/713  41.9  561/762  73.6  501/552  90.8  215/827  26.0  737/893  82.5  678/713  95.1  J  100.0  -  Combined F ^ CT X SCWL  p  CT X NH  ¥  1.  CT i n c i d e n c e 1  i ±  x  V  X P  x  I n c l u d e s e m b r y o n i c dead a n d h a t c h e d  chicks.  Table X I H a t c h a b i l i t y and l i v e a b i l i t y complex w i t h f i t n e s s .  Backcross matings.  P.. m a t i n g category  Backcross mating category  Sire  Sires  Dams  CT X SCWL '  P  l  SCWL X CT  P  l  SCWL X CT  P  l  CT X SCWL  P  l  CT X NH  P  l  NH X CT  P  l  NH X CT  P  l  CT X NH  P  l  * **  ( 1 0 w e e k s ) t o show t h e a s s o c i a t i o n o f CT gene  Dams  CT i n c i d e n c e  Hatchability  No. CT/ total  No. <c h i c k s / t o t a l iembryos  Liveability  fo  X  P  l  X  P  l  SCWL X  P  l  X  P  l  X  P  l  X  P  l  NH  X  P  l  13/155  8.4**  140/173  80.9*  CT  X  P  l  93/103  90.3**  78/112  69.6*  CT  SCWL  Combined d a t a .  CT  18/193  9.3  No. b i r d s / total chicks  177/232  76.3  159/173  91  m a t i n g n o t made 39/182  21.4**  146/215  67.9**  109/134  81  129/153  84.3**  135/163  82.8**  117/134  87  NH  20/223  9.0**  207/254  81.5**  190/203  93  CT  30/31  21/39  53.8**  20/21  95  132/139  95  96.8**  D i f f e r e n c e b e t w e e n v a l u e s t o CT a n d ST p a r e n t s s i g n i f i c a n t a t 5$ l e v e l . D i f f e r e n c e b e t w e e n v a l u e s t o CT a n d ST p a r e n t s s i g n i f i c a n t a t 1 $ l e v e l .  —  45  statistically  s i g n i f i c a n t i n c r e a s e s i n CT i n c i d e n c e were  a n i e d by s i g n i f i c a n t decreases  accomp-  i n hatchability.  A very different relationship  i s present i n t h e back-  c r o s s m a t i n g s t o b o t h p a r e n t a l s t r a i n s i n t h e CT X SCWL l i n e s . The b a c k c r o s s  to the  CT s i r e r e s u l t e d  i n progeny w i t h a  s i g n i f i c a n t l y h i g h e r i n c i d e n c e o f CT t h a n t h a t i n t h e r e c i p r o c a l backcross  t o t h e SCWL l i n e , a n d a l s o a s i g n i f i c a n t l y h i g h e r  a b i l i t y i nthe backcross backcross  of the  hatch-  o f t h e P-^ CT s i r e t o P-^ dams t h a n i n t h e  SCWL s i r e t o s i m i l a r dams.  I t s h o u l d be n o t e d  t h a t t h e P-^ CT s i r e t h a t c o n t r i b u t e d t o t h e h i g h h a t c h a b i l i t y i n the backcross  t o F-^ dams f r o m t h e CT X SCWL l i n e s i s t h e same s i r e  w h i c h c o n t r i b u t e d t o t h e o p p o s i t e r e l a t i o n s h i p when mated t o dams f r o m t h e CT X NH l i n e s . In the r e c i p r o c a l backcrosses  t o t h e SCWL l i n e t h e  m a t i n g w i t h t h e h i g h e r , i n c i d e n c e o f CT a l s o h a s t h e l o w e s t ability.  T h i s would i n d i c a t e  hatch-  that there i s nothing w i t h i n the  SCWL l i n e t h a t i s c o n t r i b u t i n g t o t h e h i g h e r h a t c h a b i l i t y h i g h CT i n c i d e n c e i n t h e b a c k c r o s s  with  t o t h e P-^ CT l i n e .  Liveability The r e l a t i o n s h i p b e t w e e n h a t c h a b i l i t y t o 10 weeks o f a g e and  CT i n c i d e n c e a r e p r o b a b l y s i m i l a r t o t h a t b e t w e e n h a t c h a b i l i t y  and  i n c i d e n c e (Table X ) .  generation. not  They a r e p a r t i c u l a r l y e v i d e n t i n t h e P^  I n s p e c t i o n o f t h e d a t a from t h e Fg g e n e r a t i o n does  show t h e same d e f i n i t e r e l a t i o n b e t w e e n l i v e a b i l i t y a n d CT  incidence.  T h i s may be d u e i n p a r t t o l e s s u n i f o r m  environmental  c o n d i t i o n s p r e v a i l i n g a f t e r h a t c h i n g and i n p a r t t o l e s s  variation  o c c u r r i n g i n t h e l i v e a b i l i t y d a t a compared w i t h t h e h a t c h a b i l i t y data. A r e g r e s s i o n a n a l y s i s a t t h e dam f a m i l y l e v e l o f l i v e a b i l i t y t o 10 weeks o n CT i n c i d e n c e showed a s i g n i f i c a n t n e g a t i v e r e g r e s s i o n was p r e s e n t  i n t h e combined t o t a l o f a l l Fg d a t a  ( r e g r e s s i o n c o e f f i c i e n t = - 0 . 3 3 6 , "t*,* = 2.18, P = 0.01-0.05 w i t h 62 d . f . ) .  The most s i g n i f i c a n t a s p e c t  of the analysis i sthe fact  that the regression i n d i c a t e s an inverse relationship  between  46  l i v e a b i l i t y a n d CT i n c i d e n c e .  The l i v e a b i l i t y  p = . 1 9 4 was t h e same a s t h o s e w i t h j? = . 3 8 8 . above v a l u e s breeding  f o r the negative  of birds  with  Therefore, t h e  regression arenot biased  by i n -  effects. L i v e a b i l i t y t o 10 weeks among t h e b a c k c r o s s p r o g e n i e s  (Table  X I ) i n t h e CT X NH l i n e s was n o t a s s o c i a t e d w i t h CT  i n c i d e n c e and remained h i g h  (93.6 - 95.2$)  from which data a r e a v a i l a b l e .  i nthe three  classes  I n t h e CT X SCWL l i n e s , a compar-  i s o n b e t w e e n t h e b a c k c r o s s p r o g e n i e s o f t h e P-^ CT s i r e a n d P-^ SCWL s i r e mated to-P-^ dams i n d i c a t e s t h a t t h e L i v e a b i l i t y , l i k e CT i n c i d e n c e  a n d h a t c h a b i l i t y , was a l s o h i g h e r  i n the  p r o g e n y o f t h e P^ CT s i r e . Discussion A hypothesis system o f m o d i f i e r s gene c o m p l e x .  has been p r e s e n t e d w h i c h p o s t u l a t e s  a  f o r f i t n e s s t h a t i s a s s o c i a t e d w i t h t h e CT  I n normal p o u l t r y f l o c k s t h e m o d i f i e r system i s  n e g a t i v e l y a s s o c i a t e d w i t h f i t n e s s and evidence h a s been p r e s e n t e d to substantiate t h i s view.  The f i x a t i o n o f t h e CT t r a i t  i n the  C a l i f o r n i a CT l i n e h a s b e e n a c c o m p a n i e d b y a r e o r g a n i s a t i o n o f the m o d i f i e r system f o r f i t n e s s so t h a t a s s o c i a t e d w i t h CT  i t h a s become p o s i t i v e l y  expression.  The t h r e e c o m p o n e n t s o f f i t n e s s t h a t h a v e b e e n regarding  evaluated  t h e i r a s s o c i a t i o n w i t h f i t n e s s have demonstrated t h a t i n  most o f t h e d a t a t h e CT t r a i t .  there  This  i s a negative  a s s o c i a t i o n b e t w e e n them a n d  i s t r u e i n t h e two UBC p a r e n t a l l i n e s a n d  t h r o u g h o u t t h e P^ g e n e r a t i o n .  The F  generation  2  generally a t the s i r e family l e v e l there o f CT w i t h f i t n e s s .  Of t h e s i x t e e n F  2  i n d i c a t e s that  i sa negative  association  s i r e f a m i l i e s , ihree  i n d i c a t e d t h a t t h e CT t r a i t h a d become c o - a d a p t e d w i t h  fitness.  The h a t c h a b i l i t y a n d l i v e a b i l i t y d a t a g e n e r a l l y i n d i c a t e a p o s i t i v e a s s o c i a t i o n b e t w e e n CT a n d f i t n e s s i n t h e s e f a m i l i e s . Chance m a t i n g o f P^ b i r d s c a r r y i n g many d i f f e r e n t h o m o l o g o u s chromosomes b e t w e e n them f r o m t h e CT l i n e p a r e n t s c o u l d favorable the P  9  lead t o  combinations of the desired modifiers f o r f i t n e s s i n  generation.  47  The t h e CT t r a i t  evidence from t h e b a c k c r o s s matings  demonstrate  that  e x h i b i t s t h e same c o - a d a p t i v e p r o p e r t i e s w i t h f i t n e s s  that i s present i nt h e parental l i n e entering the backcross.  The  evidence from t h e h a t c h a b i l i t y data i s n o t d e f i n i t i v e .  In the  CT X SCWL l i n e s , t h e b a c k c r o s s m a t i n g s  indicate  t o t h e SCWL l i n e  a n e g a t i v e a s s o c i a t i o n between h a t c h a b i l i t y and f i t n e s s . l i v e a b i l i t y data tend t o support t h i s i n t e r p r e t a t i o n .  The  On t h e  o t h e r h a n d , t h e b a c k c r o s s m a t i n g t o t h e CT l i n e i n d i c a t e s a p o s i t i v e a s s o c i a t i o n o f h a t c h a b i l i t y and l i v e a b i l i t y w i t h f i t n e s s . The  hypothesis tends t o support these r e l a t i o n s h i p s a t l e a s t i n  part.  The b e t t e r h a t c h a b i l i t y a n d l i v e a b i l i t y i n t h e b a c k c r o s s  t o t h e CT l i n e may be a c a s e o f h e t e r o s i s c a u s e d crosses.  by t h e p a r e n t a l  Why i t s h o u l d o c c u r i n t h e b a c k c r o s s t o t h e CT l i n e a n d  n o t t o t h e SCWL l i n e may be t h e r e s u l t o f c h a n c e c o m b i n i n g between t h e b i r d s used  i n t h e matings.  i s s u f f i c i e n t t o account  ability  Whether t h i s e x p l a n a t i o n .  f o r t h e magnitude o f these d i f f e r e n c e s i s  questionable. The  backcross matings  w i t h i n t h e CT X NH l i n e s  some v e r y d i f f e r e n t r e l a t i o n s h i p s . agrees w i t h the assumption  The c o - a d a p t i v e  show  evidence  t h a t f i t n e s s i s a s s o c i a t e d w i t h t h e CT  c o m p l e x b y means o f a number o f m o d i f i e r s .  The r e s u l t r e g a r d i n g  co-adaptation i n these l i n e s i s s i m i l a r t o that i n t h e s t r a i n crosses.  L i v e a b i l i t y i s good i n a l l b a c k c r o s s m a t i n g s  s h o u l d be e x p e c t e d .  H o w e v e r , t h e h a t c h a b i l i t y d a t a show t h e  exact opposite r e l a t i o n s h i p s . same CT s i r e was u s e d The  reasons  which  I t i s important t o note t h a t t h e  i n both t h e s t r a i n and breed  f o r t h i s r e v e r s a l a r e obscure.  baekcrosses.  Good h a t c h a b i l i t y i n  a l l b a e k c r o s s e s w o u l d be e x p e c t e d , b u t f o r some i n e x p l i c a b l e r e a s o n t h e h a t c h a b i l i t y i s p o o r i n t h e b a e k c r o s s e s t o t h e CT l i n e . The  importance  o f m o d i f i e r s o f t h i s type t o t h e e v o l u t i o n  of animal populations i sc o n s i d e r a b l e .  The p r e s e n t h y p o t h e s i s  shows how t h e r a p i d c h a n g e s i n a m o r p h o l o g i c a l t r a i t may o c c u r w h i l e a t t h e same t i m e , u n d e r l y i n g p h y s i o l o g i c a l p r o p e r t i e s a s s o c i a t e d w i t h t h e t r a i t may be r e o r g a n i s e d t o m a i n t a i n a s t a t e of optimal a d a p t a t i o n w i t h the  environment.  48  DISCUSSION One p r o g r a m was  of the main o b j e c t i v e s of the present  t o c a r r y out a p r e l i m i n a r y i n v e s t i g a t i o n o f p o s s i b l e  l i n k a g e r e l a t i o n s b e t w e e n c o m p o n e n t s o f t h e CT major genes. g a t i o n was  experimental  The  primary  c h a r a c t e r and  t a r g e t of t h i s phase o f the  t h e s e x chromosome b e c a u s e i t c o n t a i n e d  were a v a i l a b l e f o r s t u d y w i t h i n t h e UBC  production  investi-  genes which f l o c k s and  a l s o b e c a u s e e v i d e n c e o f s e x - l i n k a g e was  f o u n d i n 1948  c r o s s e s d u r i n g t h e d e v e l o p m e n t o f t h e CT  line in California.  U n f o r t u n a t e l y , due  known  back-  t o the i n t e r a c t i o n of genes d e t e r m i n i n g  plumage  c o l o r p a t t e r n s i n the c r o s s e s , the i d e n t i f i c a t i o n of phenotypes and  g e n o t y p e s was  part of the  behaviour  o f t h e CT  i n t h i s e x p e r i m e n t and  L e r n e r , 1949) 1.  The  i t was  necessary  t o abandon t h i s  experiment.  The obtained  i m p o s s i b l e and  complex from the  evidence  e l s e w h e r e ( H i c k s , 1953;  Hicks  and  i s summarized below: CT  gene c o m p l e x i s p r e s e n t  a t a v e r y low  incidence  i n the m a j o r i t y of p o u l t r y f l o c k s . 2.  I t i s v a r i a b l e i n i t s p e n e t r a n c e , so much so t h a t t h e  l e v e l o f CT parents 3.  i n c i d e n c e i n the o f f s p r i n g from s i n g l e matings between  o f v a r y i n g d e g r e e s o f CT The  e x p r e s s i o n o f CT  expression  i s a f f e c t e d by a p h y s i o l o g i c a l  t h r e s h o l d based upon o b l i g a t e l e v e l s of 4.  Inbreeding  i s unpredictable.  heterozygosity.  i n the absence of s e l e c t i o n w i l l i n c r e a s e  i n c i d e n c e ; w i t h s e l e c t i o n i t can l e a d t o f i x a t i o n of the 5.  Adverse environmental  incidence of 6. i n the 7.  condition w i l l  s e v e r i t y of the  expression  of the  I n b i r d s u n s e l e c t e d f o r CT  selection.  trait.  the  CT.  C o r r e l a t e d w i t h an i n c r e a s e i n CT  b e t w e e n t h e CT  increase  CT  c o m p l e x and  fitness.  i n c i d e n c e i s an  increase  trait.  there i s a negative a s s o c i a t i o n T h i s c a n be  overcome w i t h  49  8.  The  CT  t r a i t has a p o l y g e n i c s t r u c t u r e p r o b a b l y  composed o f a number o f g e n e s e a c h w i t h a m i n o r 91  In c r o s s i n g experiments,  effect.  t h e gene c o m p l e x  exhibits  s e m i - d o m i n a n c e and a p a r t i a l q u a n t i t a t i v e d e t e r m i n a t i o n i n t h e expression of the  trait.  T h e s e c h a r a c t e r i s t i c s o f t h e b e h a v i o r o f t h e CT are s i m i l a r t o those found d e t a i l e d experiments and  i n Drosophila with regard to  on p o d o p t e r a  by G-oldschmidt,  e x t r a w i n g v e n a t i o n by D u b i n i n  experiments suggest  (1948).  reported i n less d e t a i l ,  complex  the  et a l  (1951),  A number o f t h e  other  c i t e d e a r l i e r , a l s o seem t o  t h a t t h i s t y p e o f a d a p t i v e p o l y m o r p h i s m i s t o be  found  in  most c r o s s - f e r t i l i z i n g o r g a n i s m s where t h e optimum a d a p t i v e p o t e n t i a l i s d e p e n d e n t on an o b l i g a t e l e v e l o f h e t e r o z y g o s i t y . Dubinin concluded polymorphic  t h a t t h e m a i n t e n a n c e and  t r a i t s c o u l d be  behavior of  aberrant  e x p l a i n e d o n l y by a s s u m i n g t h a t  are r e l a t e d t o the adaptive genotypic p r o p e r t i e s of animal ations.  W i t h t h i s i n t e r p r e t a t i o n , t h e t r a i t s may  adaptive polymorphism. m o r p h i s m p e r se b u t  The  be c l a s s e d a s  i s a p r o p e r t y of the u n d e r l y i n g  expected,  and  a s s o c i a t e d w i t h the  t h e most d e s i r a b l e gene c o m b i n a t i o n s p h y s i o l o g i c a l processes. The  The  lower  t h a t o f ST b i r d s (1484/1592 = 9 3 - 2 $ ) . 1 d.f.). and  determining the u n d e r l y i n g  l i v e a b i l i t y t o 10 weeks o f age  significant  changing  experimental data support  a t i o n o f i n d i v i d u a l CT b i r d s was statistically  2  gener-  (466/527 = 8 8 . 4 $ ) t h a n This difference i s  ( e h i square =12.383, P = /  incidence which support  this  of the t r a i t , at l e a s t i n i t s s l i g h t phenotypic  .01  with  hatchability  assumption.  I t w o u l d be an e r r o r t o assume t h a t t h e m o r p h o l o g i c a l would have a s i g n i f i c a n t  this  i n the F  A l s o , there i s the negative regressions of  l i v e a b i l i t y on CT  trait.  t h e n , t h a t s e l e c t i o n t o i n c r e a s e t h e pheno-  t y p i c i n c i d e n c e w o u l d r e s u l t i n a l o s s o f f i t n e s s by  assumption.  popul-  adaptive p o t e n t i a l i s not the p o l y -  c o r r e l a t e d p h y s i o l o g i c a l processes I t m i g h t be  they  expression  expression,  e f f e c t on t h e f i t n e s s o f t h e  birds.  50  The CT  present  experiment  provides confirmation that  c o m p l e x e x h i b i t s s i m i l a r b e h a v i o r when i n t r o d u c e d t o a  e n t s t r a i n o f SCWL i n a d i f f e r e n t e n v i r o n m e n t .  the  differ-  A l s o , when t h e  complex i s i n t r o d u c e d t o a h e a v i e r , d u a l purpose breed which i s v e r y d i f f e r e n t from t h e l i g h t , line,  egg p r o d u c t i o n b r e e d  of the  t h e r e i s a g a i n a v e r y s i m i l a r b e h a v i o r o f t h e CT The  r a p i d l y growing  body o f evidence  f e r t i l i z i n g organisms demonstrating  CT  complex.  from c r o s s -  t h e more a d a p t i v e c h a r a c t e r -  i s t i c s o f h e t e r o z y g o t e s , p a r t i c u l a r l y t h e work o f D o b z h a n s k y ( 1 9 4 7 a , 1947b, and leaves l i t t l e  1948)  and D o b z h a n s k y and  Levene (1951),  doubt o f the f u n d a m e n t a l importance  t y p i c state t o animal populations.  of the  Conversely, animal  which  geno-  breeders  a r e o n l y t o o aware o f t h e d a n g e r s o f i n b r e e d i n g w i t h r e g a r d the f i t n e s s or adaptedness of t h e i r stock.  to  I t i s apparently  the  greater metabolic p l a s t i c i t y associated with heterozygosity which i s t h e most e v i d e n t o f t h e p h y s i o l o g i c a l p r o p e r t i e s c o n n e c t e d t h e CT  complex. Lerner  polymorphic  (1954) d e s c r i b e s p h e n o d e v i a n t s ,  t r a i t s , and  identified  I t h a s b e e n shown t h a t CT  h i g h e r i n t h e more h i g h l y i n b r e d  b e i n g any  that i s aberrant  t h e i r e x p r e s s i o n as b e i n g  with inbreeding degeneration. was  with  zygotes without  indicence there  evidence  of i n b r e e d i n g d e g e n e r a t i o n  (using hatchability  as t h e c r i t e r i o n ) .  I t w o u l d seem t h a t t h e CT  complex i s a f f e c t e d  p r i m a r i l y by h e t e r o z y g o s i s o f t h e g e n e s g o v e r n i n g t h e c o m p l e x and  p r o b a b l y s e c o n d a r i l y by t h e t o t a l i t y  of r e l a t i o n s h i p s between  genes i n t h e genotypes as e v i n c e d by t h e d i f f e r e n t r e s u l t s s t r a i n versus breed  c r o s s e s w i t h t h e CT  a m e a s u r a b l e d i f f e r e n c e t o be  observed  line. i n CT  This could  present  following  i f a r e l a t i v e l y l a r g e degree of homogyzosity  i n t h e CT  determining l o c i ;  and  if,  permit  i n c i d e n c e between  b i r d s d i f f e r i n g i n c o e f f i c i e n t s of i n b r e e d i n g under the conditions:  from  i n the  was  residual  genotype, a c o m p a r a t i v e l y h i g h degree of h e t e r o z y g o s i t y p r e v a i l e d so t h a t o t h e r components o f f i t n e s s r e m a i n e d u n a f f e c t e d .  A  genome w i t h a h e a v y c o n c e n t r a t i o n o f CT  has  d e t e r m i n i n g genes  been combined w i t h t h a t o f a d i f f e r e n t b r e e d , t h a t i s , t h e  NH  51  breed, which e x h i b i t s the elements of f i t n e s s very w e l l . p r o g e n y c o m i n g f r o m t h i s c r o s s w o u l d be heterozygous  genotypes.  The  due  e x p r e s s i o n o f CT.  2  at  to a very h i g h t h r e s h o l d f o r the  g e n e r a t i o n t h e CT  e s s e n t i a l l y t h e same f r e q u e n c y  a c o m p a r i s o n o f t h e CT  i n c i d e n c e between f u l l  with a consequently  and matings.  depressed  e x p r e s s i o n more so i n t h e f u l l  s i b matings  g r e a t e r p r o p o r t i o n o f CT p h e n o t y p e s .  But  sufficiently large  g e n e r a l l y d e l e t e r i o u s e f f e c t s from c l o s e l y  r e l a t e d m a t i n g s and The  However,  higher i n the former  t h e r e s i d u a l h e t e r o g e n e i t y o f t h e b i r d s was overcome any  present  s i b matings  g r e a t e r h o m o z y g o s i t y i n t h e more i n b r e d m a t i n g s  t h e t h r e s h o l d f o r CT  to  g e n e s were  a s i n t h e F^ p r o g e n y .  h a l f s i b m a t i n g s showed t h a t i t was The  t o have v e r y  F^ g e n e r a t i o n c o n s e q u e n t l y e x h i b i t e d  a v e r y l o w i n c i d e n c e o f CT In the F  expected  The  consequently,  h a t c h a b i l i t y remained h i g h .  h y p o t h e s i s s e t f o r t h h e r e i s t h a t CT  is a  morphological expression of i n b r e e d i n g degeneration. the experimental evidence o f h a t c h a b i l i t y and  ( f o r example, the n e g a t i v e  l i v e a b i l i t y on CT  a t i o n ) i n d i c a t e s t h a t CT  Most  of  regressions  i n c i d e n c e i n the F  i n c i d e n c e r i s e s w i t h l o s s of  2  gener-  fitness.  This i s p a r t i c u l a r l y evident i n continued consanguineous matings (Hicks,  1953). The  nature  o f the evidence  so f a r a c c u m u l a t e d i n d i c a t e s  o n l y a g e n e r a l i z e d p h y s i o l o g i c a l r e l a t i o n s h i p between h e t e r o z y g o s i t y and t h e CT histological  complex.  To f u r t h e r e m p h a s i z e t h i s p o i n t , a  study of the t i s s u e s of the lower l e g d u r i n g  d e v e l o p m e n t ( H i c k s , 1953) d i f f e r e n c e s b e t w e e n CT seems p r o b a b l e  f a i l e d t o r e v e a l any  and n o r m a l e m b r y o s .  t h a t t h e CT  c e l l u l a r or  tissue  Nevertheless, i t  c o m p l e x c a n o n l y be  maintained  extensively i n poultry flocks i f i t i s correlated with  physiol-  o g i c a l e f f e c t s t h a t are advantageous under c o n d i t i o n s of n a t u r a l selection. The  a d a p t i v e a b i l i t y o f t h e CT  complex under  artificial  s e l e c t i o n a s p r a c t i s e d i n C a l i f o r n i a by H i c k s show t h a t w i t h f i x a t i o n of the t r a i t , l o s t and  t h e e a r l y h i g h m o r t a l i t y o f CT  t h e e m b r y o n i c l i v e a b i l i t y o f CT  zygotes  embryos i s  i s as good  as  52  t h a t i n ST  zygotes.  r e a c h e d i n t h e CT  X CT m a t i n g t h e h a t c h a b i l i t y was  incidence  o f CT  good and  was  very high  matings from the manifestation  same l i n e s .  X SCWL l i n e s ,  trait  can  be  a c c o m p a n i e d by a  increased  w i t h i n the r e s i d u a l genotype. w i t h p r i m a r y e f f e c t s not  expression  o f CT  occurs i n nature.  determining  expression  but  were b a l a n c e d readjustments  An  of  a l t e r n a t e method  l o c i with  isoalleles  P r o b a b l y a c o m b i n a t i o n o f b o t h methods  importance to e v o l u t i o n a r y  theory.  d i s c u s s i o n has  I t describes  property  that could, under n a t u r a l s e l e c t i o n i n  animals,  permit populations  to adjust  a  considerable genetic  cross-fertilizing  gene c o m b i n a t i o n s t o  ing, e n v i r o n m e n t a l c o n d i t i o n s  so t h a t t h e a n i m a l r e m a i n s  adapted t o the  The  environment.  e v o l u t i o n of s e l f  p r o p e r t i e s would occur through the g r a d u a l  chang-  optimally  regulating  accumulation  stored i n the genotype.  of  Animals r e q u i r i n g a  h i g h l y heterogeneous c o n d i t i o n f o r optimal adaptation environment c o u l d  loci  differing slightly in their  This aspect of the  mutations which are  occurring  d i r e c t l y r e l a t e d to growth processes  t h e r e p l a c e m e n t o f CT  physiological effects.  the  readjustment  ( R e s i d u a l r e f e r s i> a l l t h o s e  the d i s t a l p o r t i o n of the p o s t e r i o r l i m b s ) .  within a  e a s i l y a c c u m u l a t e a l a r g e number o f  genes w h i c h i f s t o r e d w i t h i n p o l y g e n i c easily  that  genotype.  w i t h o t h e r p h y s i o l o g i c a l changes a s s o c i a t e d w i t h  e q u a l l y a f f e c t i n g CT  the  the h a t c h a b i l i t y (82.8$)  c o u l d happen i f t h e p h y s i o l o g i c a l changes  i n c o n j u c t i o n w i t h the  given  back-  I t w o u l d seem, t h e r e f o r e ,  of the r e l a t i o n s h i p s w i t h i n the t o t a l  w o u l d be  In the  a b o v e t h e p r o g e n y f r o m t h e F-^ i n t e r s e .  o f t h e CT  This  50.9$).  (84;.3$) and  break-  ( f o r example, i n  p a r e n t w i t h i n t h e CT  a l m o s t 10$  inbreeding  undoubtedly produced a general  p r o p e r t i e s of that l i n e  c r o s s m a t i n g s t o t h e CT was  h i g h average c o e f f i c i e n t of  l i n e has  down o f t h e a d a p t i v e t h e CT  The  blocks  c o u l d be  such  more  retained. Artificial  i n c r e a s e the  s e l e c t i o n f o r the morphological  frequency of i t s phenotypic expression  natural selection to readjust the a s s o c i a t e d  deviant and  the d i r e c t l y underlying  p h y s i o l o g i c a l processes.  will  cause  as w e l l  I f , under n a t u r a l  as  53  s e l e c t i o n the morphological t r a i t , p h y s i o l o g i c a l mechanisms,  rather than the underlying  become s e l e c t i v e l y a d v a n t a g e o u s , t h e n  n a t u r a l s e l e c t i o n w i l l a c t as b e f o r e t o a d j u s t t h e p h y s i o l o g i c a l processes  t o t h e new m o r p h o l o g i c a l c o n d i t i o n so t h a t t h e  m a i n t a i n s an a d a p t i v e r e l a t i o n s h i p t o i t s environment. it  w o u l d a p p e a r t h a t t h e a d a p t i v e change i n a s i n g l e  trait  animal In general,  morphological  causes only a minor a l t e r a t i o n i n the p h y s i o l o g i c a l  p r o c e s s e s , even under c o n d i t i o n s o f r a p i d change.  To t h e e x t e n t  t h a t t h i s c o u l d o c c u r , i t d e s c r i b e s a mechanism b y w h i c h r a p i d e v o l u t i o n a r y changes m i g h t be  achieved.  , even t o t h e r e o r g a n i z a t i o n o f whole  organs  54  SUMMARY  A s e r i e s o f s t r a i n and breed crosses  i nchicken  have  b e e n c a r r i e d o u t b e t w e e n t h e CT l i n e f r o m t h e U n i v e r s i t y o f C a l i f o r n i a SCWL f l o c k a n d t h e UBC SCWL a n d NH p r o d u c t i o n D a t a have been o b t a i n e d  flocks.  f r o m t h e P-^, F^, P 2 , a n d b a c k c r o s s  generations. The r e s u l t s f r o m t h e c r o s s i n g e x p e r i m e n t s h a v e d e m o n s t r a t e d t h e f o l l o w i n g c h a r a c t e r i s t i c s o f t h e CT g e n o t y p e and p h e n o t y p e : 1.  The gene c o m p l e x i s composed  o f a number o f g e n e s w h i c h  e x h i b i t semi-dominance and v a r i a b l e p e n e t r a n c e . 2.  A s CT i n c i d e n c e i s i n c r e a s e d , t h e e x p r e s s i v i t y o f t h e  defect i s increased. 3.  Incidence  ( c r i t e r i a used:  o f CT i s n e g a t i v e l y c o r r e l a t e d w i t h  evidence o f co-adaptation,  l i v e a b i l i t y ) i n unselected  flocks.  fitness  h a t c h a b i l i t y , and  I t can, w i t h  selection,  become p o s i t i v e l y c o r r e l a t e d w i t h f i t n e s s a s i n d i c a t e d i n t h e backcross 4.  m a t i n g s t o t h e CT l i n e . P h e n o t y p i c a l l y i t i s a f f e c t e d by a t h r e s h o l d  f o r e x p r e s s i o n a s i n d i c a t e d by t h e p r e s e n c e o f a  mechanism  maternal  i n f l u e n c e o n t h e i n c i d e n c e i n t h e P^ g e n e r a t i o n . 5.  Incidence  6.  The e x p r e s s i o n  animals  o f CT"is i n c r e a s e d by i n b r e e d i n g . of the defect i n c r o s s - f e r t i l i z i n g  i s c o n n e c t e d w i t h g e n e t i c b a l a n c i n g mechanisms b a s e d o n  heterozygosis. 7.  The s i g n i f i c a n c e t o e v o l u t i o n a r y p r o c e s s e s  mechanism c o n d i t i o n i n g CT e x p r e s s i o n a r e d i s c u s s e d .  of the genetic  55  REFERENCES CITED  B a t e s o n , W., and P e l l e w , C. 1920. The g e n e t i c s o f " r o g u e s " among c u l i n a r y p e a s ( P i sum s a t i v u m ) . P r o c . Roy. S o c , B, 21, 186-195. C h a s e , H.B. 1 9 5 1 . I n h e r i t a n c e o f P o l y d a c t y l y i n t h e mouse. G e n e t i c s . ^ 6 , 697-710. D o b z h a n s k y , T. 1 9 4 7 a . A d a p t i v e c h a n g e s i n d u c e d by n a t u r a l s e l e c t i o n i n w i l d p o p u l a t i o n s of D r o s o p h i l a . E v o l u t i o n , 1, 1-16. D o b z h a n s k y , T. 1947b. Genetics of n a t u r a l populations. XIV. A r e s p o n s e t o c e r t a i n gene a r r a n g e m e n t s i n t h e t h i r d chromosome o f D r o s o p h i l a p s e u d o o b s e u r a t o n a t u r a l s e l e c t i o n . G e n e t i c s , 22, 142-160. D o b z h a n s k y , T. 1948. Genetics of n a t u r a l p o p u l a t i o n s . XVI. A l t i t u d i n a l a n d s e a s o n a l c h a n g e s p r o d u c e d by n a t u r a l s e l e c t i o n i n c e r t a i n populations of Drosophila pseudoobscura and D r o s o p h i l a p e r s i m i l i s . G e n e t i c s , 33» 158-176. D o b z h a n s k y , T., a n d L e v e n e , H. 1 9 5 1 . D e v e l o p m e n t o f h e t e r o s i s through n a t u r a l s e l e c t i o n i n experimental populations of D. p s e u d o o b s e u r a . Am. N a t . , 85_, 247-264. D o n a l d , H.P. 1949. The i n h e r i t a n c e , o f a t a i l a b n o r m a l i t y associated with urogenital disorders i n pigs. J . Agric. Sci.. 164-173. 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An a n a l y s i s o f v a r i a b i l i t y i n t h e number o f d i g i t s i n an i n b r e d s t r a i n o f g u i n e a p i g s . G e n e t i c s , 19, 506-536. W r i g h t , S. 1 9 3 4 c The r e s u l t s o f c r o s s e s b e t w e e n i n b r e d o f g u i n e a p i g s , d i f f e r i n g i n number o f d i g i t s . G e n e t i c s , 1£, 537-551.'  strains  

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