Path: utzoo!utgpu!water!watmath!clyde!att-cb!att-ih!pacbell!ames!mailrus!umix!nancy!msudoc!straney
From: straney@msudoc.ee.mich-state.edu (Ronald W. DeBry)
Newsgroups: sci.bio
Subject: Re: sex ratios
Message-ID: <367@nancy.UUCP>
Date: 1 Apr 88 01:20:54 GMT
References: <1988Mar31.124839.9957@utzoo.uucp>
Sender: usenet@nancy.UUCP
Reply-To: straney@msudoc.UUCP (Ronald W. DeBry)
Organization: Michigan State Univ., Engineering, E. Lansing
Lines: 50

In article <1988Mar31.124839.9957@utzoo.uucp> rising@utzoo.uucp (Jim Rising) writes:
>I have always assumed that there tended to be a male biased sex
>ratio at conception in mammals is because the sperm carrying the 
>Y chromosome is somewhat smaller and faster than those carrying
>X. 

Hmmmm. I don't know about this one.  I'm just talking off the top of my
head here, but I'm pretty sure that (in humans, at least) vaginal pH can
have a differential effect on X vs Y bearing sperm, and that the
difference is probably large enough to swamp out a "speed" effect.  I do
know that there really is a tendency for large families to "run to
girls" or "run to boys" (it's one of the examples in Sokal and Rolf),
and I've always been suspicious that the pH effect may be the cause of
that.  I have seen news reports on using differential sperm survival in
different pHs as a way to choose the sex of a child, or at least bias
the probabilities.


>Higher early (pre-natal; juvenile) mortality may be due to 
>deleterious recessives on the X--for which the males are "homozygous."

Mammals accomplish dosage compensation by completely shutting off one X.
This happens early, at about the 8 or 16 cell stage (another guess - I
could look it up, but that will spoil the fun for those who want to
flame me for getting my facts wrong :-) ).  It *is* about the same time
that the zygote's own genes get turned on.  It could still be that some
of the deleterious effects are lessened in females, since only half of
her cells will be hemizygous for the recessive allele, but you'd have to
do some more to convince me of that.


>In birds, the females are the heterogametic sex.  I don't know much
>about ratios at conception (bet it's known for chickens), but would
>bet that there's higher female conception and early mortality.  In
>virtually all species there appears to be a biased adult sex ratio,
>with more males than females.  That, of course, increases the variance
>in male fitness--and the intensity of sexual selection--even in 
>monogamous species (because some males have no mates).
>
>--Jim Rising
>-- 

Ron DeBry  Dept. of Zoology  Michigan State University

if I knew an e-mail address that would *really* work, it would go here

BTW - since its probably my last chance (I have this feeling Duke may
kill 'em)

	      GO JAYHAWKS!!!!