Sex chromosome

Article By:

Pask, Andrew J. Department of Zoology, University of Melbourne, Melbourne, Victoria, Australia.

Last reviewed:July 2021

DOI:https://doi.org/10.1036/1097-8542.617360

Either member of a pair of chromosomes responsible for sex determination of an organism. The genome of every mammal consists of the autosomal (non-sex) chromosomes (or autosomes), of which all individuals normally carry two copies, and the sex chromosomes, which vary in number and type with the individual's sex. In mammals, the sex chromosomes are called the X and Y chromosomes. In placental and marsupial mammals, females have two X chromosomes, whereas males have one X chromosome and one Y chromosome (see illustration). (Note that monotreme mammals have a different X/Y system, which will be discussed briefly.) Some other types of animals determine sex through different chromosome systems, but the mammalian X/Y system is particularly well studied. The difference in the distribution of the sex chromosomes between males and females is a unique feature of the mammalian genome. In contrast, because all the mammalian autosomes exist in duplicate, there are two copies of all autosomal genes. This means that there is, in effect, a backup of each autosomal gene. If one gene copy is mutated, these duplicate chromosomes can undergo recombination to repair the damaged DNA. See also: Chromosome; Chromosome aberration; Deoxyribonucleic acid (DNA); Gene; Genetics; Genomics; Mammalia; Mutation; Recombination (genetics); Sex determination; Sex-linked inheritance

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