Sex Chromosomes


Sexual reproduction increases the number of different genetic constitutions that are present at each generation and that can be acted on by natural selection. In many organisms, the genetic information required for the differentiation of fertilised eggs into individuals of different sexes is contained on a special pair of chromosomes: the sex chromosomes. All sex chromosomes were derived from pairs of ordinary chromosomes (autosomes). In some instances, the autosomes acquired a gene represented by two alleles, one of which was male determining. In other instances, the presence of one of the two alleles of the sex‐determining gene on both chromosomes led to the differentiation of one sex, whereas its presence on only one of the two autosomes led to the differentiation of the other sex. In this case, the allele on the other chromosome was neutral with respect to any effect on sex determination. The autosome with the male‐determining allele or with the neutral allele accumulated mutations that inactivated most of the other genes present throughout their length. This led to a difference in the dosage of these genes between the sexes and to the evolution of mechanisms that compensate for this dosage difference.

Key Concepts:

  • In many organisms, sex is determined by the presence of specific chromosomes – the sex chromosomes.

  • These chromosomes have evolved from particular pairs of autosomes.

  • In all cases, the determination of sex is achieved by the expression of one or a few genes that are present on the sex chromosomes.

  • The expression of these genes causes the differentiation of one of the two sexes; the other sex develops by default.

  • The presence of different sex chromosomes in the two sexes can cause differences in the dosage of genes that are present on these chromosomes but that have critical functions in both sexes.

  • When such a difference in these genes would be harmful to one of the two sexes, regulatory mechanisms that compensate for the difference in the dosage of these genes have evolved.

Keywords: chromosomes; sex; sex determination; X chromosome; Y chromosome

Figure 1.

(a) A male‐dominant gene is present on the Y chromosome. (b) A female‐dominant gene is present on the W chromosome. (c) The level of product of one or more genes present on X is higher in XX individuals and leads to female differentiation. (d) Basically similar to (c) but the excess of the gene or genes present on the Z chromosome leads to male differentiation. Reproduced with permission from Graves (). © Wiley Periodicals, Inc.



Cline TW (1993) The Drosophila sex determination signal: how do flies count to two? Trends in Genetics 9: 385–390.

Disteche CM (2012) Dosage compensation of the sex chromosomes. Annual Review of Genetics 46: 537–560.

Ellegren H (2012) Sex‐chromosome evolution: recent progress and the influence of male and female heterogamety. Nature Reviews Genetics 12: 157–166.

Ford CE, Jones KW, Polani PE, deAlmeida JC and Briggs JH (1959a) A sex‐chromosome anomaly in a case of gonadal dysgennesis (Turner's syndrome). Lancet 1: 711–713.

Ford CE, Polani PE, Briggs JH and Bishop PMF (1959b) A presumptive human XXY/XX mosaic. Nature 183: 1030–1032.

Graves JAM (2013) How to evolve new vertebrate sex determining genes. Developmental Dynamics 242: 354–359.

Lucchesi JC, Kelly WG and Panning B (2005) Chromatin remodeling in dosage compensation. Annual Review of Genetics 39: 615–651.

Malone JH and Oliver B (2008) The sex chromosome that refused to die. Bioessays 30: 409–411.

Further Reading

Bachtrog D (2013) Y‐chromosome evolution: emerging insights into processes of Y‐chromosome degeneration. Nature Reviews Genetics 14(2): 113–124.

Bachtrog D, Kirkpatrick M, Mank JE et al. (2011) Are all sex chromosomes created equal? Trends in Genetics 27(9): 350–357.

Dupont C and Gribnau J (2013) Different flavors of X‐chromosome inactivation in mammals. Current Opinion in Cellular Biology 25: 314–321.

Li X (2011) Sex chromosomes and sex chromosome abnormalities. Clinics in Laboratory Medicine 31: 463–479.

Mank JE (2013) Sex chromosome dosage compensation: definitely not for everyone. Trends in Genetics 29(12): 677–683.

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How to Cite close
Lucchesi, John C(Apr 2014) Sex Chromosomes. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0005791.pub2]