When the sex of the baby is determined by the female which produces two types of gametes having different sex chromosomes, it is known as female heterogamety.
Female heterogamety is observed in females of some birds, insects, and fishes.
Example of female heterogamety:
The female butterfly consists of ZO sex chromosomes. (Z is the female chromosome).
It is the presence or absence of Z that determines the sex of the offspring.