Question

In birds, the male is the homogametic sex. Females are ZW. A dominant sex-linked allele, Z^S gives silver feather color. Z^s gives gold. Mate a silver female with a gold male. How can you identify male and female offspring by feather color alone?

Answer 1

The male birds are homogametic, meaning that they have identical sex chromosomes. The female birds are heterogametic and hence have non-identical sex chromosomes. Thus the males are ZZ while the females are ZW.

Now, we have a sex-linked gene, where the allele ZS codes for silver feather and the allele Zs codes for golden feather and ZS is dominant over Zs.

The mating is performed between a silver female and a golden male. The silver female will be ZSW and the golden male has to be homozygous recessive and hence ZsZs since the heterozygous condition would give rise to silver feather as ZS is dominant over Zs.

Thus the cross is performed between ZsZs males and ZSW females. We can analyze the cross using a Punnett square as follows:

	ZS	W
Zs	ZSZs	ZsW
Zs	ZSZs	ZsW

As can be seen, the offsprings are 50% females and 50% males. All the male offsprings are genotypically ZSZs and all the female offsprings are genotypically ZsW. Thus the females will produce golden feathers, while the male offsprings being heterozygous will produce silver feathers. Thus based on the feather of the offsrpings, their sex can be determined.