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Sex chromosome abnormalities are far more common than abnormalities of the other 44 chromosomes (the autosomes) because they rarely produce deadly diseases. There is a very simple reason for this: the numbers of X and Y chromosomes vary between ordinary men and women (women have two Xs, men have one X and one Y), and so the precise number of X and Y chromosomes in the cells of an individual cannot be essential to life.
For example, women have no Y chromosome at all; and so, apart from genes involved in making testicles etc., the Y chromosome consists almost entirely of so-called “junk DNA."
In contrast, since both men and women have an X chromosome, there is no evolutionary pressure opposing the presence of genes essential to life here, and indeed several are present. For example, the genes for producing the clotting factors that allow wounds to stop bleeding are on the X chromosome (the absence of the correct forms of these genes causes haemophilia).
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Abnormalities in Females
Sex chromosome abnormalities in females are due to a variation in the number of chromosomes. For example;
If only one X chromosome is present, the individual has Turner Syndrome and their genotype is X0. 98% of fetuses with Turner syndrome are spontaneously aborted, but Turner Syndrome nevertheless accounts for about 1 in every 2000 live births. Turner Syndrome can cause a range of symptoms, but short stature and infertility are almost always present.
Triple X syndrome occurs when a woman has three X chromosomes. Such women tend to be towards the upper end of the normal range of heights and the lower end of the normal range of intelligence, but the condition causes no major symptoms. It is exquisitely rare, but not entirely unknown for women to have more than three X chromosomes.
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Abnormalities in Males
Sex chromosome abnormalities in males are due to irregular numbers of either X or Y chromosome or both. For example;
The genotype XXY causes Klinefelter’s Syndrome. The presence of an extra X chromosome inhibits the development of the male reproductive system, and so almost all affected males are effectively sterile. Other symptoms sometimes include a generally androgynous appearance and gynecomastia (development of breasts). Some degree of learning impairment may or may not be present. The condition accounts for about 1 in every 500 live births in the US.
The genotype XYY is present in about 1 in 1,000 live births in the US. In general it causes no symptoms, and 97% of those carrying an extra Y chromosome are unaware of the fact. XYY men have a slightly higher than average height (estimated at 6’2’’) and a national survey conducted in 2004 for the US Centre for Disease Control estimated that 10% of XYY boys have a learning disability. It was formerly believed that they had an increased tendency towards criminal behaviour and this has been used (largely unsuccessfully) as a defence in murder trials. But it is now believed that XYY males are no more likely to commit crime than other men of similar intelligence.
More bizarre chromosomal combinations, such as XXYY, have been identified, but these are incredibly rare and little can reliably be said about them.