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Magic Oddball [soylentnews.org] writes:

Our ancestors originally had 24 pairs of chromosomes, the last with 700 genes each until they shed enough to combine with the 23rd pair, forming our modern sex chromosomes. The new X stabilized at 1,100 genes, but Y continued to shrink and now has a mere 27 genes. While any absent/damaged X genes in one parent can be filled in with the other parent's good copy, males inherit their father's Y chromosome directly, and any mistakes in transcription are thus passed on to the next generation.

The popular theory is that Y will keep shrinking until it combines with another chromosome, which will then take over its task. A much more optimistic theory has come out of UC Berkeley [berkeley.edu]:

The human Y, however, may be done shrinking, says Wilson Sayres...the lead author of a new analysis that suggests the Y isn't shrinking, it's honed. It has one job and one job only. ...
"The few genes that are left on the Y, if you lose them, you have big problems ... Sperm don't swim, their heads are malformed, they can't fertilize an egg."

Sayres feels this explains why Y chromosomes around the world have only 1/10 of the genetic variation other chromosomes do. The old "Genghis Khan" theory was that a few men produced far more offspring than others did, but a complex computer DNA simulation proved that there's too much variation in the human genome for it to work. In other words, we're the product of only the best Y-chromosome genes on a huge variety of men — not primarily the historic aggressors nature might consider the 'best' at reproducing.

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