sex chromosome

sex chromosome A chromosome whose presence or absence is linked with the sex of the bearer, and that plays a role in sex determination. It is present in all sexually reproducing, diploid animals and plants. The sex that has a homologous pair of sex chromosomes in the nucleus is said to be homogametic (referred to as ‘XX’), whereas the sex with a dissimilar pair or with an unpaired chromosome, is said to be heterogametic (referred to as ‘X’ or ‘XY’). The homogametic sex produces gametes that are identical in their chromosome sets, all containing one X-chromosome. The heterogametic sex produces equal numbers of two different types of gametes, one with one X-chromosome, and one without (possessing either a Y-chromosome or none at all). Union of gametes of the two sexes thus results in equal numbers of offspring of the two sexes. In mammals and many other animals the presence or absence of a Y-chromosome determines sex; in birds and reptiles, and in some fish, amphibians, and insects, sex is determined by the number of X-chromosomes in relation to the number of autosomes. Occasionally a mutation will produce an extra dosage of X-chromosomes in the gamete, and so in the progeny (e.g. in Klinefelter's syndrome in humans, the individual genotype (a male) possesses an extra X-chromosome (XXY).