sex-linked
adj. describing either a gene that is located on one of the sex chromosomes, typically the X chromosome (X-linked), or a trait determined by such a gene. The process whereby a sex-linked gene or trait is passed on from parent to offspring is called sex-linked inheritance. Sex-linked inherited diseases from a defective gene on the X chromosome include hemophilia, itself called X-linked recessive because the defective gene is usually a recessive allele. It is carried by females and expressed mostly in their male offspring. In their daughters, who have two X chromosomes, the defective gene is usually masked by the normal, dominant allele on the other X chromosome, whereas in their sons, who have an X and a Y chromosome and in whom the Y chromosome lacks many of the alleles of the X chromosome, masking cannot occur. Disorders that are X-linked dominant, in which only one copy of an allele of a defective gene on the X
chromosome is sufficient to cause inherited disorder, may occur in either male or female offspring depending on which parent is affected. A form of sex-linked inheritance in which a recessive trait is inherited from father to son by way of a single gene on the Y chromosome (Y-linked) affects only male offspring. Diseases or disorders passed on through Y-linked inheritance are rare.