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Sex and the Single Y: Our Unpaired Genes Aren’t Disappearing, They’re Just Pared Down

March 20, 2014
by Brendan Buhler

Fear not for genetic machismo: Reports of the human Y-chromosome’s demise are greatly exaggerated. Over time, it may have evolved to be tiny, but it is not failing, according to UC Berkeley geneticist Melissa Wilson Sayres. Rather, it has been whittled down to a lean, mean, male-making machine.

The case for the death of the Y has always been plain: its evident diminishment. Humans have about 20,000 genes spread over 46 chromosomes collected into 23 pairs. Twenty-two of those pairs are different copies of the same chromosome. The 23rd pair is, potentially, mismatched. These are our sex chromosomes, the X and the Y. Now compare the two human sex chromosomes. They started off the same size, with roughly 700 genes apiece. The X now has about 1,100 genes. And the Y, the root of masculinity? Twenty-seven genes. Not 2,700 or 270—27, like a dead rock star. It’s just this shriveled little nub.

A statue of a soldier

Even more: The Y has disappeared before. The X, too. That is to say, our ancestors used to have a 24th pair of chromosomes. Those were our sex chromosomes until they shrank down and attached themselves to a previously identical pair of chromosomes that we now call the X and the Y. The evidence is right there—both the X and the Y have a stretch of genes left over from when they merged with a different pair of non-sex chromosomes.

The Y’s problem is that it suffers from the very flaw that sexual reproduction solves. Consider asexual reproduction. An organism creates offspring using only its own genetic code—it clones itself. The problem is that in the copying, it’s possible to leave out bits or make mistakes, which are then included in the next copy, and then that copy makes another mistake, which is passed down as well, and so on until the whole package becomes a lumpen mess that is caught in the gene pool’s filter.

Sexual reproduction solves this problem. You get two copies of each of your genes, one from mom and one from dad. In you, these copies combine and the shabby bits in one copy are usually fixed by the other. Much sturdier. Plus, you get to have sex.

The Y, alone among our chromosomes, is always reproducing asexually. A man doesn’t get a copy of the Y from each parent, just the one Y from dad, whose Y was copied from granddad’s. So while all the other pairs of your chromosomes combine when they replicate (even the X, which gets to combine in women), the Y is just off copying itself. So it makes errors, it drops bits and it never really has a chance to accrue positive traits. It’s like an old sci-fi plot: clone madness!

(The commercial banana has the same problem, in that every banana tree is a clone. Delightfully, the problem of the Y is the problem of the banana writ small.)

So, the argument goes, the human Y chromosome will keep rotting away until its functions are, once again, subsumed into another chromosome.

The human Y, however, may be done shrinking, says Wilson Sayres, who is the Miller Postdoctoral Fellow in the Department of Integrative Biology and 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 Y is, one might say, the Ferrari of sex chromosomes. It does not haul the kids to soccer practice, it does not shop for groceries, it does not tow a boat to some stupid lake. The Y chromosome, it is sleek, it is fortissimo, it is machismo, it is …

It is really more of a Model T, Wilson Sayres says. The Y chromosome is a jalopy: It has wheels, an engine, and seats. It has what it needs to have to qualify as a car and nothing more. If anything else falls off of the Y, it ceases to function.

“The few genes that are left on the Y, if you lose them, you have big problems,” Wilson Sayres says. “Sperm don’t swim, their heads are malformed, they can’t fertilize an egg.”

There’s something else that’s weird about the Y, too. All of the Y chromosomes in the world are startlingly similar. The Y chromosome has about one tenth the genetic variation that occurs on all other chromosomes.

One way to explain this is what Wilson Sayres calls the Genghis Khan theory: A very few men are much more reproductively successful than all the others. (The theory takes its name from the 13th-century Mongolian warlord famous for the extent of his conquests and also for the extent of his conquests. Some scientists estimate that 16 million men are descended from the mighty Khan.)

The problem with applying this theory to the entire human population is that it doesn’t seem to agree with life as we live it—it’s like imagining that your town’s mayor has droit du seigneur. Wilson Sayres, in fact, has the math to prove the Genghis Khan theory wrong. (For the mass of humanity. Obviously, it worked out pretty well for ol’ Genghis.)

Starting with the DNA of eight African men and eight European men, Wilson Sayres ran a computer simulation regressing 4,000 generations backward in time. The result? There was too much variation in the entire genome for the story of human reproduction to have been all Genghis Khan all the time. All in all, it looks as if three men reproduce for every four women.

And yet we’re left with a Y chromosome that is considerably less diverse than it seems like it ought to be. The reason, according to Wilson Sayres’ simulations, is that the Y has become so slimmed down that there isn’t anything left that it can lose without producing sterile men. Instead of the more familiar, competitive, survival-of-the-fittest type of natural selection that we tend to think of, the Y seems to be subject to a purifying selection where it either works or it doesn’t. And when Wilson Sayres runs her simulations 4,000 generations into the future, the Y looks unlikely to change.

Should humanity last another million years, the Y will still be around much as it is today, Wilson Sayres says. “You can call me on it if I’m wrong.”

Brendan Buhler aspires to be a hobby farmer. In the meantime, his freelance writings appear in the LA Times, Bay Nature, Modern Farmer, and California.

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