Since the discovery of the Y chromosome and its general state of health, so to speak, there have been observations/expectations/musings that the Y chromosome may be on its way out... eventually. Why? Let's take a step back - we have 23 pairs of chromosomes; 22 of these pairs are termed of chromosomes are termed autosomes - whether you're male or female, we've all got them. The 23rd pair - the sex chromosomes - determine whether we're male (XY) or female (XX). The XY system of sex determination exists in all mammals*. In birds, its slightly different - females are the heterogametic sex (ie females have the different sex chromosomes, and their sex chromosomes are call ZW), with males being ZZ (homogametic). Originally though, all these sex chromosomes started life as autosomes. As sex specific genes accumulate on a chromosome, recombination between that particular pair of chromosomes has to stop (in this case the X & Y chromosomes) - otherwise you end up with problems when genes that are specific/necessary for males end up in females. Another consequence of this, is that as recombination has stopped, genes on the Y chromosome that get hit by mutation have no way of "getting fixed". What do I mean by that? Well if you think about a gene on the X chromosome that gets screwed up (or on any of the other autosomes for that matter), recombination means that the screwed up copy of the gene doesn't last very long - it can be recombined out (remember X chromosomes will recombine with each other in females). But with the Y chromosome, this is not an option. What happens, is that the regular genes that were present before it became a sex chromosome (ie before it became a sex chromosome and recombination was stopped), get hit by mutation until they no longer function - the Y chromosome loses genes in this way. The reduced size of the Y chromosome, compared to the X chromosome (Y chromosome is on the right in the picture).

The thinking was, that over time, we would expect the Y chromosome to deteriorate to extinction. But one of the main findings of these studies on the evolution of the Y chromosome is that the genes that were lost, were lost millions of years ago, and that the remaining genes on the Y chromosome are essential and unlikely to be lost for this reason; the gene loss on the Y chromosome has stabilised over evolutionary time (millions of years). Also interesting is that these 12 remaining genes are expressed in all cells, yet they are somewhat different to the counterpart genes on the X chromosome - there is a difference throughout males and females (and not just the obvious ones - doh!) due to these regulatory genes. Another step closer to understanding the secrets encoded in our genomes....

*I've over simplified here - platypus actually have 10 sex chromosomes, and there also exist other sex determining systems - wikipedia "sex determination system" for more details.