Posted by:
Paul Hollander
at Fri Jan 16 18:25:51 2004 [ Email Message ] [ Show All Posts by Paul Hollander ]
>Does anyone happen to know if both the W and Z sex chromosomes have information on them? I was really shocked to hear that the Y chromosome doesn't (or at least not info that is used). I would like to understand more about how this absence of the information from a 2nd X chromosome creates a male in humans. I wonder if the W in snakes to create a female (WZ) is a similar thing or if both sex chromosomes in snakes are used. It would help us know what sort of things to look out for as possible sex linked genes in snakes. If a mutation was on the W it would only ever be seen on females and males could not even carry it. If it was on the Z it might be dominant in females but perhaps recessive, codominant, or dominant in males or even seen in spots on the males.
From what I've found, boa constrictors (maybe all Boidae) do not have different sized sex chromosomes. Colubrids do have different sized Z and W chromosomes, but the W isn't as small as the mammalian Y chromosome. And there was an article on the evolution of the Y chromosome in Scientific American in the last few years. As I recall, it said that there was originally information on the Y chromosome that was slowly lost over millions of years. So I'd assume that there could be information on snake W chromosomes.
My impression is that the mammalian Y chromosome is required to make a male. As I recall, an X human is a poorly developed female, XY is male, and XXY is also male.
Birds have a large Z and small W sex chromosomes. A male is ZZ, and a female is ZW. My understanding is that sex is additive in birds, with one Z producing a female and ZZ producing a male.
The only possible candidate I know of for a sexlinked mutant in snakes is brindle in the black rat snake. And that hasn't been proven to my satisfaction.
I've messed around a little with genetics of pigeons and ringneck doves. Both have sexlinked mutants on the Z chromosome. Pigeons, for example, have three alleles at the b locus. In males, the mutant brown is recessive to both ash red and normal, and the mutant ash red is dominant to both normal and brown. A male pigeon that has an ash red mutant gene paired with a normal gene is just ash red, not with patches of ash red and patches of normal coloration. And we can tell what the female's genotype is at the b locus from her appearance. If snake sexlinkage is analogous to pigeon sexlinkage, a sexlinked mutant could be dominant, codominant, or recessive to normal in a male snake. And I'm not expecting an analog of calico in snakes.
BTW, here's another bit of genetics terminology: hemizygous. It means that a gene is present on one chromosome but has no match on the other chromosome. So a brown female pigeon is hemizygous brown because it has only the one brown gene on the Z chromosome and no match on the W chromosome.
Paul Hollander
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RE: NERD, more info on calico retic please - Paul Hollander, Fri Jan 16 18:25:51 2004