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Stardate 20011107.1313 (On Screen): Someone linked to me from a CNN discussion board, and what they linked to is my essay on how the genetic veriability in the human Y chromosome proves that the Noah story is untrue. In that same posting, I found this link, which partially explains why it is that the Y chromosome is so small. But it leaves something out.

What it shows is that the Y chromosome is uniquely destined for genetic damage. What it doesn't explain is why it's small. They leave out an interesting bioengineering fact: the race is to the swift. Selection of sperm cells is based on speed, and all other things being equal, lighter cells have an edge. (This is a good thing; it means that sperm cells which, because of a mistake in meiosis, carry an extra chromosome are heavier and are unlikely to win the race and thus form a deformed child.) That's why a slight majority of births are male: sperm cells carrying Y chromosomes weigh slightly less than sperm cells carrying X chromosomes. So for every hundred girl babies born there are about 103 boys. (But by the time you reach age 6 the numbers have equalled out, because boy babies also have a slightly higher death rate.)

According to this, originally the Y chromosome was an X chromosome carrying a critical mutation. Over time, it begins to accumulate damage due to the fact that it ceases to recombine. But if there were physical damage to Y chromosomes during sperm formation, so that some of them were physically smaller than others (because they'd physically dropped sections of genetic material which were unimportant anyway) then those sperm cells would be lighter and faster than the ones with the full sized chromosomes. Thus not only is there a long term trend towards genetic damage on the Y, but also towards decreasing its size.

But this can only happen for the Y chromosome because it is the only chromosome which cannot ever double in a child. (There isn't any way that both parents can carry Y chromosomes.) A Y chromosome invariably is matched to an X which carries all the missing information the Y lost. (discuss)

There is only one way a YY egg could form, and it would require simultaneous mistakes in meiosis by both parents. First, the mother would have to create an egg with no X chromosome. Second, the father would have to produce a sperm cell with two Y's. Such a sperm cell would still weigh less than one carrying an X chromosome so it wouldn't be out of the race. If such a sperm cell fertilized a normal egg then you'd get an XYY, the notorious "supermale". If it fertilized our X-less egg, you'd get a YY. But a YY egg would not be viable; there is essential genetic information on the X chromosome which would not be expressed and the fetus would die early.