In humans, females inherit an X chromosome from each parent; males inherit their X chromosome from their mother and their Y chromosome 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.