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X Marks the Spot for Male Fertility

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X and Y: From Then to Now

For those who are a little rusty on Genetics 101, a brief review may be in order. Each normal cell in the human body has 46 chromosomes: 22 pairs of autosomes -- "ordinary" chromosomes that are identical in men and women --and two sex chromosomes. Women have two Xs, and men have one X and one Y, and that's usually enough to make all the difference.

But 300 million years ago, when our ancestors were still crawling around swamps on their bellies, there were no sex chromosomes.

"It turns out that once upon a time, the X and the Y were the same. They were the two members of a perfectly ordinary pair of autosomes," Page said at a recent Whitehead seminar. "I will argue that 300 million years ago, when we were reptiles, we had males and females, we existed as males and females. The males made sperm, the females made eggs, but we didn't have sex chromosomes, we only had ordinary chromosomes, and our sex was likely determined -- whether we as a reptilian embryo developed as a male or female -- by the temperature at which we, as an egg, incubated."

Since that time, however, as the human race worked its way up the evolutionary ranks, it seems that the genes that control spermatogenesis -- the creation of sperm -- have become so essential to the species that they've been copied over and over again and shuffled from the garden-variety autosomes into the "newly" created sex chromosomes.

"You started with an ancestral gene in flies and worms that was required for spermatogenesis. That then got duplicated in higher animals. And then very recently in Old World monkeys and humans, there was a duplication onto the Y, and that Y gene itself was multiply duplicated," says Steven A. Wassermen, PhD, professor of biology at the University of California at San Diego. "What appears to have happened in humans in particular is that you moved these genes onto the Y chromosome that are responsible for spermatogenesis and therefore subject to sexual selection.

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