This just in! What I learned today:
I was doing some reading on cat genetics and I learned female mammals undergo X chromosome inactivation!
In each of their cells they randomly inactivate one X chromosome to prevent overexpression of the X chromosome. That's how you get the magnificent tortie cat. In some pigment cells the Orange chromosome is active and in the others the black is active.
So that made me wonder, do male birds (with the ZZ sex chromosome arrangement) do Z chromosome inactivation to prevent overexpression of the Z chromosome? Sexlinked colors do seem to work differently in birds.
And I discovered, NO!
This really does explain a lot.
Take Sex-linked Dilution, for example.
Female geese have one Z allele so they have one copy of the dilution, therefore, they are a slightly lighter gray than wild type and have a little white.
Heterozygous male geese have a copy of the dilution and a copy of wild type. Since they have one dilution allele like the female geese, they are the color of the female geese.
But Homozygous males have a double expression. They have two copies of the dilution and they are mostly white with a little bit of light grey.
You see the same in faded pigeons. Hemizygous females and heterozygous males are the same color, Homozygous males are lighter.
In barred chickens the same thing, dark hemizygous females and heterozygous males, light homozygous males.
But before you get too comfortable there is a twist.
There is another goose gene that is so rare I forgot to show a photo of it in my last post where I said I showed all possible goose alleles. (Actually, I believe it might not actually be all that rare and might just be the "weird swan goose white" I mentioned.)
Aqua Discovered by Snyder's Waterfowl.
Hemizygous females and homozygous males are white. Heterozygous males have this blue spotted color.
A more common example would be ash red pigeons. Hemizygous females and homozygous males are ash red but heterozygous males are mostly ash red with a few blue/black flecks.
Why is it like this? Incomplete dominance, I guess wild type and white want to battle it out, I honestly don't know.
And of course recessive sexlinked genes like Chocolate or Dilute pigeons need the bird to be hemi or homozygous to express. It really makes me curious about what other genes are sexlinked in mammals and birds and how they looked.
