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:
- 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 .
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).
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.
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