In many species, each sex has a sex-specific size, vocal cartilage, and musculature. A female mammal has a vagina, cervix, uterus, oviducts, and mammary glands. A male mammal has a penis, seminal vesicles, and prostate gland. Secondary sex determination affects the bodily phenotype outside the gonads. Moreover, as we shall see, both diverge from a common precursor, the bipotential gonad. In mammalian primary sex determination, there is no “default state.” The formation of ovaries and testes are both active, gene-directed processes. For a complete ovary, a second X chromosome is needed. Furthermore, an individual with only a single X chromosome and no second X or Y (i.e., XO) develops as a female and begins making ovaries, although the ovarian follicles cannot be maintained. A person with five X chromosomes and one Y chromosome (XXXXXY) would be male.
Unlike the situation in Drosophila (discussed below), the mammalian Y chromosome is a crucial factor for determining sex in mammals. This factor organizes the gonad into a testis rather than an ovary. The Y chromosome carries a gene that encodes a testis-determining factor. If the egg receives another X chromosome from the sperm, the resulting individual is XX, forms ovaries, and is female if the egg receives a Y chromosome from the sperm, the individual is XY, forms testes, and is male. The male, being XY, can generate two types of sperm: half bear the X chromosome, half the Y. Since the female is XX, each of her eggs has a single X chromosome. Every individual must have at least one X chromosome. In most cases, the female is XX and the male is XY. In mammals, primary sex determination is strictly chromosomal and is not usually influenced by the environment. Primary sex determination is the determination of the gonads.
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