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).

sex chromosome A chromosome whose presence or absence is linked with the sex of the bearer, and plays a role in sex determination. It is present in some sexually reproducing plants (see SEX DETERMINATION). 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 2 different types of gametes, one with one X-chromosome, and one without (possessing either a Y-chromosome or none at all). The union of gametes of the 2 sexes thus results in equal numbers of offspring of the 2 sexes. Most plants and a minority of animals are hermaphrodite (i.e. they contain the sexual organs of both male and female). But many higher plants, fungi, and bacteria are dioecious (i.e. have separate sexes), and in these cases sex is most commonly determined by sex chromosomes. Many of the higher plants have the male gender as the heterogametic sex (e.g. Melandrium, Humulus, and Rumex), but, as with the animal kingdom, sometimes the female can be the heterogametic sex (e.g. Fragaria).

sex chromosome A chromosome that operates in the sex-determining mechanism of a species. Many animals have two different types of sex chromosome. For example, in mammals there is a large X chromosome and a much smaller Y chromosome; the female has two matching X chromosomes, making it the homogametic sex, whereas the male has one X chromosome and one Y chromosome, making it the heterogametic sex. In other animal groups, such as butterflies and moths, birds, and reptiles, the situation is reversed: males have two matching W chromosomes, while females have one W chromosome and one Z chromosome. Sex chromosomes carry genes governing the development of sex organs and secondary sexual characteristics. They also carry other genes unrelated to sex (see sex linkage). See sex determination; testis-determining factor.

sex chromosome n. a chromosome that is involved in the determination of the sex of the individual. Women have two X chromosomes; men have one X chromosome and one Y chromosome. Compare autosome.

sex chro·mo·some • n. a chromosome involved with determining the sex of an organism, typically one of two kinds. Also called heterochromosome.