I 5.7 A chicken's comb is determined by gene interaction between two loci. (a) A walnut comb is produced when there is a dominant allele at each of two loci (R_P_). (b) A rose comb occurs when there is a dominant allele only at the first locus (R_pp). (c) A pea comb occurs when there is a dominant allele only at the second locus (ppR_). (d) A single comb is produced by the presence of only recessive alleles at both loci (rrpp). (Parts a and d, R. OSF Dowling/Animals Animals; part b, Robert Maier/Animals Animals; part c, George Godfrey/Animals Animals.)

independent assortment, these single-locus ratios can be combined by using the multiplication rule: the probability of obtaining the genotype RrCc is the probability of Rr (/2) multiplied by the probability of Cc (72), or %. The probability of each progeny genotype resulting from the testcross is:

Progeny Probability genotype at each locus

Overall probability rrCc rrcc

Phenotype red peppers brown peppers yellow peppers green peppers

When you work problems with gene interaction, it is especially important to determine the probabilities of single-locus genotypes and to multiply the probabilities of genotypes, not phenotypes, because the phenotypes cannot be determined without considering the effects of the genotypes at all the contributing loci.

Another example of gene interaction that produces novel phenotypes is seen in the genes that determine comb shape in chickens. The comb is the fleshy structure found on the head of a chicken. Genes at two loci (R, r and P, p) interact to determine the four types of combs shown in 4 FIGURE 5.7. A walnut comb is produced when at least one dominant allele R is present at the first locus and at least one dominant allele P is present at the second locus (genotype R_P_). A chicken with at least one dominant allele at the first locus and two recessive alleles at the second locus (genotype R_pp) possesses a rose comb. If two recessive alleles are present at the first locus and at least one dominant allele is present at the second (genotype rrP_), the chicken has a pea comb. Finally, if two recessive alleles are present at both loci (rrpp), the bird has a single comb.

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