Sometimes, in fact, sex is observed before birth. Most commonly, this happens via ultrasound imaging of the fetus. Less commonly, it is possible to look at the karyotype—a visual representation of fetal chromosomes, organized roughly by size—which has been obtained through the usefully diagnostic but somewhat risky mid-pregnancy procedure known as amniocentesis.
All mammals have “Genetic Sex Determination,” which means that we have chromosomes dedicated to starting us down the path of maleness or femaleness. They are called sex chromosomes, in contrast to the autosomes which comprise most of our genetic makeup, and which do not vary predictably by sex. A tiny number of mammals—the echidnas, and the duck-billed platypus—have several pairs of sex chromosomes. The remaining several thousand of us mammals, however—everything from bats to koalas, kangaroos to whales—all the many thousands of other species of mammals have just one pair of sex chromosomes in each of our cells. Humans are mammals, so we have Genetic Sex Determination. Humans are neither echidnas nor duck-billed platypi, so we have just the one pair of sex chromosomes.
The number of chromosomes in each of our cells varies between species. Ocelots and margays have 18 pairs of chromosomes, for instance, while most other cats have 19 pairs1. Most of the great apes, including chimps, have 24 pairs of chromosomes, but humans have only 23. That is: humans have 22 pairs of autosomes, and at that 23rd position: one pair of sex chromosomes.
Humans have 23 pairs of chromosomes in almost all of our cells. Gametes—sex cells—are a notable exception to this2, however, having only 23 chromosomes each, instead of 23 pairs. If you’re female, your gametes are called eggs; if you’re male, they’re called sperm. If successful (as the vast majority are not), an egg or a sperm will combine with a gamete of the other type and make a new life. As such, so as not to create a new life with double the chromosomes of their parents, gametes have halfthe chromosomal complement of somatic (body) cells: one copy of chromosome 1, one copy of chromosome 2, etc., all the way down to chromosome 23.