Hello,

Here is the full explanation given below. Just go through it:

In birds, females are the heterogametic, as females have one Z and one W chromosome (ZW) and can therefore produce two types of gametes (Z / W oocytes). By contrast, males are ZZ  & can produce only one type of gamete (homogametic)—the Z-bearing sperm. In XY and ZW systems, the homologous chromosomes are genetically unequal due to the suppression of homologous recombination and accumulation of deleterious mutations on one chromosome of the heterogametic  .In the XY system, it is the Y that genetically degenerates; in the ZW system, it is the W. Today, the mammalian X carries over three times more genes than the Y does, whereas the chicken Z carries over ten times more than the W.