In chickens, the black color of the plumage dominates over the brown, and the presence of the crest over its absence. A brown crested rooster is crossed with a black hen without a crest. In their offspring, half of the chickens are black crested and half are brown crested. What are the genotypes of the parent hen and rooster?
Let’s designate the gene that causes the black plumage of chickens as B, then the gene for brown feathers will be written as b.
Let us designate the gene that determines the development of the crest in chickens as C, then the gene that, in homozygous form, leads to its absence will be c.
A chicken without a crest for this trait is necessarily homozygous – cc, and is capable of transmitting to chickens only the gene for the recessive trait c. Since all her chicks have crests, it should be concluded that the rooster is homozygous for the dominant gene.
Thus, the cock is bbCC, the spermatozoa it produces are bC.
Since half of the chickens are brown (bb), and the other half are black (BB / Bb), therefore, the black hen is heterozygous for the gene for feather coloration and homozygous for the gene for the absence of crest – Bbcc. It produces eggs Bc and bc.
The offspring of such a rooster and hen will have the following options:
black crested chickens (ВbСс) – 50%;
brown crested chickens (bbСс) – 50%.
This confirms the results indicated in the problem statement.
Answer: rooster – bbcc, chicken – bbcc.