Question

In many species of birds, males are XX and females are XZ. With birds like this, who is most likely to display a sex-linked recessive trait?

Answer 1

In birds, sex-linked recessive traits are more likely to appear in females since they have a single Z chromosome to express such traits, being hemizygous.

Step-by-step explanation:

In birds, sex determination operates on a ZW system, where males are homozygous (ZZ) and females are heterozygous (ZW). This is contrary to the mammalian XX/XY system. With this sex determination system in birds, females are more likely to display a sex-linked recessive trait because they are hemizygous—meaning they have only one copy of the Z chromosome that could carry such a trait.

Therefore, if the Z chromosome carries a recessive trait, there is no second Z chromosome to mask its expression, as would be the case with males (ZZ) or mammalian females (XX). Thus, sex-linked recessive traits in birds will commonly appear in females who have only one allele for the trait on their Z chromosome.