Females typically have two X chromosomes, while males typically have one X and one Y chromosome. In every cell of female mammals, there are two X chromosomes, but one of them is inactivated. This inactivation stops transcription from occurring, ensuring that a potentially toxic double dose of X-linked genes does not occur. The X chromosome is about three times larger than the Y chromosome, containing about 900 genes, while the Y chromosome has about 55 genes. Biologically female individuals inherit an X chromosome from their father and the other X chromosome from their mother. Biologically male individuals always inherit their X chromosome from their mother.