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

In humans, sex determination is primarily driven by the presence or absence of the Y chromosome at the time of fertilization.12

Genetic sex is established when a sperm cell, carrying either an X or a Y chromosome, fertilizes an egg, which always carries an X chromosome. Individuals with an XY complement are typically genetic males, while those with an XX complement are genetic females.12

The Y chromosome plays a critical role by carrying testis-determining genes that direct undifferentiated embryonic gonads to develop into testes. Once formed, these testes secrete hormones, such as testosterone and Müllerian inhibiting hormone, which drive the development of male reproductive structures and the regression of female precursor ducts. In the absence of these Y-linked signals, the embryo follows a female developmental pathway.2

The X chromosome is also essential for normal development; specifically, two X chromosomes are required for the development of normal ovaries. To balance gene expression, females undergo X inactivation, a process where one X chromosome in each somatic cell becomes transcriptionally silent.23