Gender is determined genetically by two sex chromosomes. In mammals these are the X and Y chromosomes, with males having XY chromosomes and females having XX. In a mating, the parents each contribute one sex chromosome. As a result, it is the male that, genetically, determines the gender of the offspring. In birds, however, the sex chromosomes are referred to as Z and W, with females having ZW chromosomes and males having ZZ. In birds, therefore, it is the female that determines the gender of the offspring.