A cross is performed between a bird that is homozygous for red feathers and a bird that is homozygous for blue feathers. Purple offspring result due to incomplete dominance. Then, two of the purple offspring are crossed. What proportion of the offspring from the second cross will be purple?

Let the red-feather allele be R and the blue-feather allele be B. The original cross produces purple offspring with genotype RB, so crossing two purple birds is RB × RB. The offspring genotypes are RR, RB, RB, and BB; two of the four are purple, so the proportion is 2 in 4, choice B.

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