Question

Barred feather pattern is a Z-linked dominant trait in chickens. What offspring would you expect from (a) the cross of a barred hen to a non-barred rooster? (b) the cross of an Fl rooster from part (a) to one of his sisters?

Answer 1

Let the "barred" allele be caller B and the "non-white" allele b. Since chickens use chromosomes
`Z` and
`W` to determine sex, hens would have chromosomes
`ZW`, and roosters would have chromosomes
`ZZ`. A Z-linked gene is represented as a superscript on the
`Z` chromosome,
`Z^(B)` for the dominant allele and
`Z^(b)` for the ressesive allele.

A barred hen would have a copy of B on its Z chromosome, a non-barred rooster would have both copies of b on both Z chromosomes. Using Punnet squares to represent the crosses we get the following cases:

`\begin{center}\begin{tabular} \ & Z^(B) & W \\ \ Z^(b) & Z^(B)Z^(b) & Z^(b)W \\ \ Z^(b) & Z^(B)Z^(b) & Z^(b)W \\ \end{tabular}\end{center}`

That is a ratio of two barred heterozygote roosters to two non-barred hens. Crossing them we get:

`\begin{center}\begin{tabular}c\ & Z^(b) & W \\ \ Z^(B) & Z^(B)Z^(b) & Z^(B)W \\ \ Z^(b) & Z^(b)Z^(b) & Z^(b)W \\ \end{tabular}\end{center}`

That is a ratio of one barred heterozygote rooster to one barred hen to one non-barred rooster to one non barred hen.