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Intersex conditions can also give rise to other combinations, such as XO or XXX in mammals, which are still considered as female so long as they do not contain a Y chromosome, except for specific cases of testosterone deficiency/insensitivity in XY individuals while in the womb.
However, these conditions frequently result in sterility.
Most mammalian females have two copies of the X chromosome as opposed to the male which carries only one X and one smaller Y chromosome (but some mammals, such as the platypus, have different combinations).A distinguishing characteristic of the class Mammalia is the presence of mammary glands.The mammary glands are modified sweat glands that produce milk, which is used to feed the young for some time after birth. Mammary glands are most obvious in humans, as the female human body stores large amounts of fatty tissue near the nipples, resulting in prominent breasts.A Y sperm and an X egg produce a male, while an X sperm and an X egg produce a female.The ZW sex-determination system, where males have ZZ (as opposed to ZW) sex chromosomes, is found in birds, reptiles and some insects and other organisms.