The Sex Chromosomes

 
The human genome has twenty three chromosomes, the autosomes, plus either two X chromosomes, or an X or a Y. In the discussion so far, we have treated the chromosomes as pairs of double spirals of matching molecules, which is the case with the first twenty-two, the autosomes. The sex chromosomes do not match, however, the X chromosome is much longer than the Y, and their genetic sequences are different.

Although, they are referred to as the 'sex chromosomes', it turns out that most of the X chromosome appears to be concerned with features that are not to do with sexual dimorphism per se. Common conditions due to errors of the X chromosome, for instance, are colour blindness and hemophilia. The Y chromosome contains, it would appear, mostly non-coding material.

Most of the genes concerned with sexually dimorphic features, including testes or ovaries, occur on the autosomes of both sexes. It is during development that different combinations of genes are active or inactive, due to factors both internal and external to the organism. For instance, hormone balance can affect genetic regulation of the distribution of fat around the body. Transsexuals take estrogen pills to encourage an increase of fatty tissue around the breast area. Gynecomastia occurs naturally in some teenage boys and middle aged men, to the former's great embarrassment, since we don't talk about 'that sort of thing.'

While the autosomes are paired, the X and Y chromosomes are not. Moreover the X chromosome is much larger.
 

Both X and Y chromosomes have a short length which pairs during meiosis, called the pseudo-autosomal region. These are sometimes referred to as the short arms of the X and Y chromosomes respectively. The long arms are the remainder, from the centromere onwards.

In female mammals, one of the two X chromosomes in each cell, is inactivated, being compressed into what is known as a Barr body. For a while, the presence of a Barr Body was as used as a test for the sex of competitors in the Olympic Games. This process apparently occurs at random, so that some cells and their progeny have the maternal X chromosome, while the others have the paternal X.

The phenomenon can be seen in tortoiseshell cats, which are always female. It seems that a gene implicated in determining hair colour is carried on the X chromosome. If the cat were male it would be one colour or the other, since it would have only one X chromosome. The theory is that if the parents are differently coloured, the random inactivation of the female's X chromosome gives rise to random patches of fur of different colour.

 
However, some genes on the X chromosome escape inactivation, notably some that have copies on the Y chromosome, especially in the pseudoautosome, and, mysteriously, some on the long arms also escape. It would seem that two active X chromosomes are needed at some time early in gestation, otherwise the internal genital organs, particularly the oocytes, do not develop, as in Turner's syndrome.

A large part of the long arm of the Y-chromosome appears to be what are called tandem repeats, lengths of repeated sequences of non-coding DNA. Since the length of these sequences is extremely variable from one individual to another, they have been used for tracing male pedigrees and population migration.

Our present understanding is that there is just one gene on the Y chromosome that could be said to be specifically sexual - the SRY gene. It is very small, only a few hundred base pairs. We will discuss this gene more fully in the next section.

References

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