Humans possess 46 chromosomes, but only two—X and Y—govern biological sex. Females carry two X chromosomes (XX), while males carry one X and one Y (XY). The Y chromosome houses genes essential for male development.
During fertilization, the sperm determines the embryo’s sex. If the sperm carries an X chromosome, the resulting XX embryo develops as a female. A Y‑containing sperm yields an XY embryo that initiates male development.
The Y chromosome contains the sex‑determining region Y (SRY), which, around week 7 of gestation, activates testis formation. The emerging testes secrete hormones such as testosterone, driving the development of male secondary sexual characteristics.
Alterations to the Y chromosome are typically not life‑threatening, but they can impair male development and fertility. In rare cases, embryos inherit additional Y chromosomes. Current research suggests that males with an extra Y may exhibit increased body size and learning challenges.
