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

Abstract

The crooked toes defect in chicken is an example of an aberrant polymorphic trait associated with inbreeding degeneration. The mechanism governing its expression is a property of balanced genotypes based on obligate levels of heterozygosity. Fixation of the trait was accomplished in California by a selection program combined with inbreeding in a Single Comb White Leghorn flock. The crooked toes line developed from this program was combined, in the present investigation, in crosses with genetically unrelated stock. Strain and breed (New Hampshire) crosses to the crooked toes line were made reciprocally and carried through the F₁, F₂ and backcross generations. Offspring from each cross were mated from the F₁ generation to establish four lines based on reciprocal parental matings. Comparisons were then made to determine the levels of incidence, the degree of phenotypic expression, the association of the trait with fitness and the effects of sex-linkage, maternal environment and inbreeding. Crooked toes were found to be a polygenic trait characterized by semi-dominance and variable penetrance. The trait is associated with a number of modifiers affecting fitness. Inbreeding per se increased incidence, and increased incidence was accompanied by increased expressivity. The behavior of matings between parents with varying degrees of crooked toe incidence was unpredictable. The appearance of the defect is determined to a considerable degree by the presence of a physiological threshold for its expression. An unsuccessful attempt was made to locate marker genes associated with aggregates of crooked toe determining loci.

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