sex chromosome

Chromosome that differs between the sexes and serves to determine the sex of the individual. In humans, whether a person is male or female is determined by the combination of the two sex chromosomes in the body cells. In females both chromosomes are the same – two X chromosomes (XX). In males the two chromosomes are different – one X chromosome and one Y chromosome (XY). The Y chromosome is shorter than the X and contains fewer genes. A single gene on the Y chromosome determines sex.

As a result of meiosis gametes from a female each contain one X chromosome. However, gametes from a male are of two kinds. Half of the gametes contain an X chromosome and half contain a Y chromosome. If an X-carrying gamete from a male fertilizes a female gamete the result will be a female. If a Y-carrying gamete from a male fertilizes a female gamete, the result will be a male.

While the other 22 pairs of chromosomes, known as autosomes, can exchange genetic material by recombination, the X and Y chromosomes have grown apart over the course of human evolution and largely lost the ability to exchange material. This has led to a slow degeneration of the human Y chromosome.