Re: Science fact question: Why 'Y'?

Dan Goodman wrote:
Brenda Clough wrote:

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.

Okay -- then why do male birds have the equivalent of the X, and
females the equivalent of the Y?
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





Somebody has to have a bigger genetic package than the other. Once you set the pattern, it just keeps on going. (If you are going to have a very small investment, your sperm are quicker and lither if they are smaller than the next guy's. Meanwhile, back at the X ranch, if a a big egg/long gestation makes my offspring bigger and healthier, then eggs will tend to get bigger or gestation longer.

Brenda


--
---------
Brenda W. Clough
http://www.sff.net/people/Brenda/

My novel REVISE THE WORLD is now appearing at
www.bookviewcafe.com
.

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