Two genes, C and I, which undergo independent assortment, affect feather color in leghorn chickens. Each gene has two alleles, one of which is dominant for feather color. The gene product of the dominant allele C is necessary for color in the feathers; however, the gene product of the dominant allele I is an inhibitor of color production in the feathers, even when C is present. What ratio of white feathers : colored feathers is expected among the progeny of the cross Cc Ii × Cc Ii?
A)9:7
B)10:6
C)12:4
D)13:3
E)15:1

Question

Quizplus · Accepted Answer

In this genetic cross, the presence of the dominant allele I inhibits color production regardless of the C allele's presence. Therefore, to have colored feathers, a chicken must have at least one dominant C allele (for color) and be homozygous recessive for the I allele (ii) to not inhibit color. The cross Cc Ii × Cc Ii results in a 9:3:3:1 ratio for the four possible phenotypic combinations, but since three of these combinations result in white feathers due to the presence of the dominant I allele, only the combination where the genotype is ccii will result in colored feathers. This gives us a 13:3 ratio of white to colored feathers.

Two Genes, C and I, Which Undergo Independent Assortment, Affect