A special chromosome concerned with the determination of sex. In mammals there are two sex chromosomes called X and Y. Males have the genotype XY, females XX. All eggs therefore bear one X chromosome, but sperms may bear either one X (in which case the sperm will give rise to a daughter) or one Y (in which case the sperm will give rise to a son). The male sex is therefore referred to as heterogametic, the female as homogametic. Birds have a very similar system, except that males are homogametic (the equivalent of XX) and females are heterogametic (the equivalent of XY). Genes carried on sex chromosomes are called 'sex-linked'. This is sometimes confused with 'sex-limited', which means having expressions in one sex or the other (not necessarily carried in sex chromosomes).
