How do chromosomes determine gender?

Gender is determined by the X and Y chromosomes, where XX denotes female and XY denotes male.

In humans, gender is determined by the 23rd pair of chromosomes, also known as the sex chromosomes. Each person has 46 chromosomes in total, 22 pairs of autosomes and one pair of sex chromosomes. The sex chromosomes are different in males and females. Females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males have two distinct sex chromosomes (XY), and are known as the heterogametic sex.

The Y chromosome carries the SRY gene, which triggers male development. If an embryo has XY chromosomes and the SRY gene is present, it will develop into a male. If the SRY gene is not present, as in the case of XX chromosomes, the embryo will develop into a female. The SRY gene initiates the development of testes in an embryo, which then produce male hormones leading to the development of male secondary sexual characteristics.

It's important to note that while the presence of the Y chromosome and the SRY gene generally leads to male development, there are exceptions. For example, individuals with Androgen Insensitivity Syndrome (AIS) have XY chromosomes but their bodies are insensitive to male hormones, so they develop female secondary sexual characteristics. Conversely, individuals with Congenital Adrenal Hyperplasia (CAH) have XX chromosomes but their bodies produce an excess of male hormones, leading to the development of male secondary sexual characteristics.

In summary, while the X and Y chromosomes generally determine gender, it's the presence or absence of the SRY gene and the body's response to sex hormones that ultimately lead to the development of male or female characteristics. This is a complex process involving many genes, hormones, and biological pathways.