CHROMOSOME KEY

SEX differences occur on a number of independent dimensions. Genetic sex, or the organization of the "sex chromosomes," is often thought to indicate the true sex, with males having one X and one Y, while females have two Xs. However, about one in 400 people are neither XX nor XY, but have less common combinations, such as XO (a single X), XXY, or even different combinations in different cells within the same individual, for instance some cells XX and some cells XY.

During gestation, the sex chromosomes determine the differentiation of the gonads, usually into ovaries or testes, but sometimes into ovo-testes (combining ovarian and testicular cells), and occasionally the gonads are absent. For the first six weeks, all fetuses have essentially female genitals. Testosterone produced by fetal testes causes the clitoris to grow to form a penis, the inner labia to wrap around the penis to create the penile urethra, and the outer labia to fuse to create a scrotal sac. The process may not complete, resulting in genitals which may look nearly female, but with a large clitoris; nearly male, with a small penis and perhaps with the urethral meatus (urinary aperture) along the bottom rather than at the tip of the penis; or they may be truly "right in the middle" ambiguous genitals, with a structure that might be considered either a large clitoris or a small penis, surrounded with what might be a split, empty scrotum, or outer labia, and with a small vaginal pouch that opens into the urethra rather than into the perineum.

Because different hormonal systems control the differentiation of the genitals and of the internal reproductive organs, some individuals are born with nearly male genitals outside, and a uterus, tubes, and ovaries inside. Some are born with female genitals outside and undescended testes inside.