What determines sex in humans at the genetic level?

Sex in humans is determined by the pair of sex chromosomes, specifically the presence or absence of the Y chromosome.

In humans, sex is determined by the 23rd pair of chromosomes, also known as the sex chromosomes. There are two types of sex chromosomes: X and Y. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

The process of sex determination begins at fertilisation. Each egg cell from the mother carries one X chromosome, while the sperm cell from the father can carry either an X or a Y chromosome. If the sperm cell carrying an X chromosome fertilises the egg, the resulting zygote will have two X chromosomes and develop into a female. If the sperm cell carrying a Y chromosome fertilises the egg, the zygote will have one X and one Y chromosome and develop into a male.

The Y chromosome carries the SRY gene, which is the sex-determining region of the Y chromosome. This gene triggers the development of male physical traits. If the SRY gene is present (as in XY individuals), the embryo will develop testes and male secondary sexual characteristics. If the SRY gene is absent (as in XX individuals), the embryo will develop ovaries and female secondary sexual characteristics.

It's important to note that while the presence or absence of the Y chromosome generally determines sex, there are exceptions due to variations and mutations in the sex chromosomes or the genes they carry. These can result in conditions such as Klinefelter syndrome (XXY), Turner syndrome (XO), and Androgen Insensitivity Syndrome (XY individuals who are phenotypically female). These exceptions highlight the complexity of sex determination and the role of genes beyond the sex chromosomes.