How It Works: XX and XY Determine Sex

How It Works: XX and XY Determine Sex

Humans have 46 chromosomes arranged in 23 pairs. The first 22 pairs are autosomes, identical in both males and females. The 23rd pair — the sex chromosomes — differs between the sexes. Females carry two X chromosomes (XX); males carry one X and one smaller Y chromosome (XY).

During meiosis, the sex chromosomes separate into gametes. Because a female has XX, all her eggs carry a single X chromosome — she can only contribute X. A male has XY, so during meiosis he produces two types of sperm in equal numbers: sperm carrying X and sperm carrying Y. The sex of a child is therefore determined entirely by which type of sperm fertilises the egg. If an X-bearing sperm fertilises the egg, the resulting XX zygote develops into a female. If a Y-bearing sperm fertilises the egg, the XY zygote develops into a male.

Because X- and Y-bearing sperm are produced in equal numbers, the theoretical probability of a male or female birth is exactly 50:50. However, this is a probability — in practice, families with several children of the same sex are not uncommon, because each conception is an independent event.