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. consider both the illustration and the text. compare the inheritance system of bird with that of humans.