I'm not saying that because I'm a sexist, but because that was what I was taught in my biology class =[
But if it's wrong, then it's wrong.
HOLD THE PHONE GUYS. ITS WIKI TIME.Synopsis:
The Y Chromosomes and X Chromosomes split off from a common Chromosomal Predecessor our distant ancestors had, and the X and Y have continued to diverge due to the same kind of pressures that drive Speciation. X and Y can't recombine like other genes, or the offspring would get all screwy and wouldn't survive, so Y Chromosomes evolved to not be able to recombine with X Chromosomes... and since only 1 Y tends to exist per viable male, Y Chromosomes can't benefit from natural selection the same way the 2 X Chromosomes can in women.
Unlike the X Chromosome, there are no vital genes on the Y Chromosome. Y Chromosomes also mutate
way faster than any other genes in the body due to how one sperm cell (the only place you find Y Chromosomes in most viable adult humans) divides repeatedly, accumulating mutations with each division. That, and the testes are not in as stable and protected an environment as ovaries, which exposes them to even more mutation.
Because Y Chromosomes tend to accumulate mutations, and are unable to recombine as X Chromosomes are, they are unable to benefit from the "Natural Selection" that usually occurs in genes. This has caused Y Chromosomes to accumulate junk and harmful DNA, and "degrade" with each generation. In several mammals with short reproductive cycles, this has led to the extinction of the Y Chromosome already.
However, mammals losing their ancestral Y Chromosomes
does not mean the extinction of the Male Mammal. In animals where the Y Chromosome has already disappeared, the "Maleness" genes have just migrated to other genes, and species have evolved new ways of determining sex. Humans are estimated to lose our Y Chromosomes entirely, and evolve some new method of Sex Determination, around 14 Million years from now.