Monday, 14 August 2006

Y is it so?

It is a question that has plagued humankind’s deepest philosophical thinkers ever since men were men and women were women: Why are men and women really so different? Most women claim to have long noticed the strangeness in males, but now it seems that at last scientists have the evidence.

Researchers studying chromosomes, which harbour genes containing DNA that act as your human body blueprint, have discovered that the Y chromosome, which determines maleness, is downright strange.

Chromosomes are found in every one of the more than one billion cells that make up the human body and are inherited from your parents. There are 23 pairs of chromosomes in the body, one of which determines sex. Women have a XX pairing of sex chromosomes, and men a XY pairing. Research reported in Scientific American suggests, not only is the Y chromosome tiny in size compared to the X, but that it holds far less genetic information. There are less than 50 genes on the Y compared to about 3000 on the X. Moreover, all other pairs of chromosomes have exactly the same number of genes.

However, the tiny Y has had an exciting journey to its current form. The research has found that Y did not start out its life as the runt of the chromosomes. About 300 million years ago, before mammals and even most dinosaurs had appeared on the Earth, a fascinating process of evolution started to change the Y chromosome, which was then identical to the X. The sex of our ancestors was then determined, not by sex chromosomes, but by the temperature of the embryo at a developmental stage before birth. This is similar to modern reptiles.

The Y chromosome began its independent life when the first mammals appeared. It contained a gene called SRY for “sex-determining region Y.” This gene triggers the formation of the testes, which then produce testosterone and other substances that mould maleness. The presence of this gene on Y however disrupted the DNA surrounding it such that the X and Y chromosomes could no longer entirely pair up in a process called recombination. Recombination allows chromosomes to swap genetic information during the production of sperm and egg cells. Without recombination, which keeps the chromosomes fresh from possibly harmful genetic mutations, the Y chromosome progressively mutated so much so that much of it no longer exists and 95 percent of the X and Y chromosomes does not recombine.

But does this mean that as evolution takes its course the ever-shrinking Y will spell the death of the male?

Geneticists say that we should not yet add the male to the endangered species list, and for now all is not lost for the little modern Y. It was previously thought that the Y chromosome contained mostly junk DNA and that the production of the testes triggered by SRY was its only function. But it is now known that the genes remaining on the Y chromosome are particularly important for survival in males and for fertility. In about half of all couples affected by infertility the problem rests with the man. Disruption of the genes on Y can reduce sperm count causing infertility. Infertility research is now directing efforts towards understanding the strange Y and searching for a cure. On the flip side, the possibility of new male contraceptives that target the sperm producing regions of Y is being developed.

So things aren’t looking to bad for us blokes.

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