Syzygy23 said:
Several people have mentioned that the Y chromosome is 'breaking up' or 'beginning to fail'. What are you people going on about? First I've heard of a specific chromosome going sour worldwide.
An individual can only ever have one copy of a Y chromosome, from their father. That means that if something goes wrong with a gene, the damaged gene can't be repaired from another chromosome, which is what happens in the X chromosome.
Now consider the fact that each Y chromosome has been suffering these sorts of mutations for millions of years, ever since we diverged from chickens (I think). Any one object lasting that long is going to be incredibly damaged unless you repair it along the way. All this DNA costs energy to make, so if you're producing a load of useless 'junk DNA', then you're wasting your energy, and evolution tends to kill off individuals who do that. So the Y chromosome shrinks, to the point where it is now nearly useless.
Wikipedia [https://en.wikipedia.org/wiki/Y_chromosome#Shrinking_theory] has a good overview of this, I've probably got a few things wrong there.