Each human cell contains 23 pairs of chromosomes that carry DNA within their nucleus. The X and Y chromosomes are one such pair. They determine the biological sex, reproductive organs, and sexual characteristics that develop in a person. Female (XX) mammals inherit one X chromosome from each parent, but males (XY) receive an X from their mother and a Y sex chromosome from their father.
Diseases and conditions found only on the X chromosome are said to be sex-linked, as are rare Y-linked conditions such as male infertility. The X chromosome is larger and confers more regulatory functions throughout the brain and body. While disease such as color-blindness and hemophilia have long been understood to be X-linked, biologists and behavioral geneticists now know that important brain functions and conditions, from intelligence to autism, may be up regulated or down regulated by genes found on the X chromosome.