How One Chromosome Sets the Usual Pattern
Human egg cells carry one X chromosome. Sperm cells carry either an X or a Y, because the chromosome pairs in the father’s cells are separated as sperm are made. When a sperm fertilizes an egg, its chromosome combines with the egg’s X: an X-bearing sperm usually produces an XX embryo, while a Y-bearing sperm usually produces an XY embryo.
That is why, for the usual XX-or-XY pattern, the sperm’s chromosome determines the baby’s chromosomal sex. The mother does not choose which kind of sperm fertilizes the egg; which sperm succeeds is a chance event.
From Chromosomes to Development
Chromosomes are packages of DNA, and their genes help guide development. A Y chromosome usually carries the SRY gene, which can start a pathway that leads the developing gonads to form testes. Hormones produced during development then influence the formation of reproductive anatomy. Without that pathway, development generally follows a different course.
Chromosomal patterns and bodies do not always fit neatly into XX and XY categories: variations in chromosomes, genes, or hormone responses can alter development. But in the most familiar pattern, the decisive difference begins with whether the fertilizing sperm brings an X or a Y—one chromosome arriving in a single cell at the start of development.
