Out of the 46 chromosomes humans have, only two are responsible for determining the sex of an individual. They are the X and the Y chromosomes. Females carry only the X chromosome, while males carry both X and Y. The X and Y chromosomes carry different genes, and the Y chromosome contains the genes that code for male attributes.

All female eggs carry one X chromosome; therefore it is the chromosome carried in the sperm that determines the sex of the embryo at the moment of conception. If the egg is fertilized by a sperm with an X chromosome, the embryo develops into a female with an XX genotype. When the sperm carries a Y chromosome, a male embryo results, with an XY genotype.

The genetic information within the Y chromosome contains the instructions to develop the embryo into a male; in particular, the chromosome has an area called the sex determining region Y (SRY). Until about week seven of a pregnancy, a male embryo and a female embryo are no different. Around this time, genes trigger the formation of the testes in a male embryo. The newly formed testes begin producing hormones, such as testosterone, that further contribute to the physical characteristics of a male.

Abnormalities in the Y chromosome are generally not life-threatening or debilitating. A defect may result in incomplete male development and infertility. Additionally, it's possible for an embryo to receive two or more Y chromosomes. Although researchers are still studying this condition, males with extra Y chromosomes may be larger than average or have learning disabilities.