Re: Science fact question: Why 'Y'?

pullo wrote:

I was trying to give an 9 year old the simplified version of how a baby's gender was determined - the XX and XY chromosomes thing with a little bit about how the genes on the part of the X chromosome that the Y doesn't have can cause recessives to be expressed in males.

Then he asked a question that I couldn't answer:

Why?

Why does the male have the Y?


It seems like a negative trait that evolution would select against. Is there any positive aspect of having a Y?

Ant thoughts on why this sort of biology would develop?


You are not asking, are you, why there is sex? The mixing of genes via sex works for a more diverse and healthier population of descendants. There are very few single-sex animals. They can't evolve as fast.

You are asking a strategy question. The female strategy is to invest heavily in the egg, which is quite large and has quite a lot of stuff in it. There is a limit to how many eggs a female can produce. The male's sperm is essentially a swimming dab of DNA, but is therefore much less costly to manufacture. So you make a large big bet, or many cheaper smaller ones.

This is similar to the question, shall I have many many offspring that I cannot take especial care of, the way mosquitoes do, or should I have one or two that I nuture carefully, like elephants? By and large this tactic is set by your species -- you don't see elephants spawning like flies. But even within the human population, you can see some groups trending towards one tactic and others to the other. If you live in an area with high infant mortality it is to your advantage to have many kids, so that a couple may survive to adulthood. In an affluent first-world culture you do better to have one or two children and save to send them to Yale.

Brenda

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Brenda W. Clough
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