20 June 2003
ByAppeared in BioNews 213
Scientists have unveiled a detailed analysis of the human Y chromosome, the genetic material that contains the 'male' switch. When flipped on, this switch (a gene called SRY) makes an early embryo develop into a baby boy. For many years, this was thought to be the Y chromosome's one important feature, and that the rest was 'a genetic wasteland'. But two new studies, published in last week's Nature, confirm that other important genes are found on the Y chromosome. The new studies also show that the Y chromosome is able to swap around and 'fix' its own genetic material: something scientists had always assumed it could not do, since it is not inherited in pairs like all the other human chromosomes.
The mammoth effort, lead by David Page of the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, reveals that 95 per cent of the Y chromosome is 'male specific', with the remaining five per cent shared by the other sex-determining chromosome, the X chromosome. (In humans, females inherit two X chromosomes, while males inherit one X and one Y chromosome). The researchers determined the sequence of 23 million DNA base pairs of this male-specific region, and identified 78 genes, which together produce 27 different proteins. Eleven of these appear to be switched on only in the testes, suggesting they are crucial to male fertility.
The two studies also showed that the Y chromosome has the ability to fix itself, as it has duplicate, mirror images of the regions that carry important genes. While other chromosomes swap genetic material within their pairs every time new egg and sperm cells are produced, the solitary Y chromosome uses a process known as 'gene conversion' to swap material around between its duplicated regions. This means that it has a way of purging damaged genes, so is not on a downward evolutionary spiral, as previously thought. 'We have a new way of understanding how the rotting tendencies of the Y are counteracted' said Page. But he added that the repeated regions were 'a double-edged' sword', as they are also susceptible to deleting themselves out altogether, which could lead to male infertility.