rising@utzoo.uucp (Jim Rising) (03/31/88)
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. Higher early (pre-natal; juvenile) mortality may be due to
deleterious recessives on the X--for which the males are "homozygous."
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
--
Name: Jim Rising
Mail: Dept. Zoology, Univ. Toronto
Toronto, Ontario, Canada M5S 1A1
UUCP: {allegra,ihnp4,linus,decvax}!utzoo!risingstraney@msudoc.ee.mich-state.edu (Ronald W. DeBry) (04/01/88)
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!!!!
cs4l3az@maccs.UUCP (....Jose) (04/04/88)
In article <1988Mar31.124839.9957@utzoo.uucp> rising@utzoo.uucp (Jim Rising) writes: >>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. Actually, the "shutting off" of the X chromosome isn't of such importance... approximatley half of the female's body cells will have the maternal X chromosome inactivated, the other half having the fraternal X inactivated; thus any cell products which cannot be supplied by one set of cells can be supplied by another. Also, those genes located on the tips of the X chromosome are not even deactivated to begin with. The genetic basis of the approximately 1:1 male:female sex ratios is still not fully understood and is the subject of intense study; that the answer lies simply in how fast the sperm swim, or the vaginal pH is unlikely. (The vaginal pH DOES affect fertilization, apparently by way of differences in antigens found on the surfaces of X sperm and Y sperm). ----------------------------------------------------------------------- "Fighting for Truth, Justice ....Jose Hachezero and whatever might seem like Department of Biochemistry fun at the time" cs4l3az@maccs ------------------------------------------------------------------------