Sex chromosomes

Sex is determined by the sex chromosomes 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:

• Klinefelter syndrome is caused by the presence of more than one X chromosome when a Y chromosome is also present (47,XXY). In this case the Y chromosome is enough to determine maleness. They will have 1 barr body in their chromosomes due to the presence of 2X chromosome. However, the affected male will be sterile and may also be affected by other symptoms such as the development of breasts, small external genitalia and testes and also mental impairment. Males affected also tend to be phenotypically tall.
• Turner syndrome is caused by monosomy of the X chromosome (45,X). The affected female will probably not sexually mature properly and will oftern experience spontaneous abortion. Those affected also tend to be phenotypically short .