In humans, females inherit an X chromosome from each parent; males inherit their X chromosome from their mother and their Y chro
mosome from their father. The same is true of all other placental mammals: males produce X and Y gametes, and females produce only X gametes. In this system, referred to as the XX-XY system, maleness is determined by sperm cells that carry the Y chromosome. Yet the XX-XY sex determination system does not occur in all animal species. It is only one of a variety of systems within the animal kingdom. Above are Punnett squares illustrating three examples of gender inheritance patterns: humans, insects and birds. Consider both the illustration and the text. Compare the inheritance system of bird with that of humans.
In bird, females are heterogametic, produce two type of gametes. The two different sex chromosomes of a female bird has been designated to be the Z and W chromosomes. Whereas males have a pair of Z- sex chromosomes. Males are homogametic produce only one type of gametes with Z-sex chromosomes. Thus sex determination is ZW-ZZ type. In humans sex determination is XX-XY type. Males are heterogametic produce X and Y gametes and females are homogametic produce one type of gametes. Therefore, in bird sex are determined by female gametes.
