How do X and Y chromosomes influence biological sex in humans?

In humans, biological sex is determined at the time of fertilization by the combination of sex chromosomes inherited from the parents.12

Every human egg carries one X chromosome, while sperm cells carry either an X or a Y chromosome. An XX combination typically results in a genetic female, while an XY combination results in a genetic male.12

The Y chromosome is critical for male development because it contains testis-determining genes that direct undifferentiated embryonic gonads to become testes. These testes then secrete hormones, such as testosterone and Müllerian inhibiting hormone, which promote the development of male reproductive structures and the regression of female precursors. Conversely, the presence of two X chromosomes is necessary for the development of normal ovaries, and in the absence of male-specific hormonal signals, the fetus develops along the female pathway.2

While this process is standard, variations in chromosomal distribution can lead to atypical sexual development, such as XXY or XO constitutions.3