Sex chromosome

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Sex is determined by the sex chromosomes, also known as allosomes, of a particular organism. In humans, females have 2 X chromosomes whilst males have XY chromosomes. The female (in humans and many other mammals) is known as the homogametic sex, whilst the male is known as the heterogametic sex. By contrast some organisms (birds and some reptiles, butterflies and moths) the male is homogametic and the female is heterogametic. In this case the sex chromosomes are represented by WZ (female) and ZZ (male) rather than the XX-XY mechanism.

There are many different disorders and diseases associted with sex chromosomes resulting from non disjunction during meiosis:

A pair of sex chromosomes in a diploid organism is the only pair in which the chromosomes are not morphologically similar homologs.There are two chromosomes responsible for determinig sex of an organism. In most organisms these are the X and Y chromosomes. In humans and most other animals, females are the  homogametic sex as they inherit two X chromosomes (XX), and males are the heterogametic sex as they inherit one X chromosome and one Y chromosome (XY). However in some organisms such as birds, moths, butterflies, some reptiles and some fish, the opposite applies, meaning that the male is the homogametic sex (XX) and the female is the heterogametic sex (XY).[3]

The inheritance of different sex chromosomes comes about as the homogametic parent always passes on an X chromosome in their gametes and the heterogametic parent will either pass on an X chromosome (producing a homogametic embryo) or a Y chromosome (producing a heterogametic embryo). Roughly half the gametes produced by the heterogametic parent will contain an X chromosome and roughly half will contain a Y chromosome, meaning that the ratio of males to females in the offspring will be roughly 1:1.[4]


  1. [Daniel L. Hartl and Elizabeth W. Jones (2009) Genetics analysis of genes and genomes (seventh edition) mississauga, Jones and Bartlett Publishers, Inc.]
  2. Genetics Home Reference, (2014). Klinefelter syndrome. [online] Available at: [Accessed 14 Nov. 2014]
  3. Hartl, D. L., Ruvolo, M. (2011) pp131-136 Genetics: Analysis of Genes and Genomes, 8th ed., Burlington: Jones & Bartlett Learning, LLC
  4. Hartl, D. L., Ruvolo, M. (2011) pp131-136 Genetics: Analysis of Genes and Genomes, 8th ed., Burlington: Jones & Bartlett Learning, LLC
1. Gu L, Walters J. Evolution of Sex Chromosome Dosage Compensation in Animals: A Beautiful Theory, Undermined by Facts and Bedeviled by Details. Genome Biology and Evolution. 2017;9(9):2461-2476.
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