Question about Heterozygous Genes?

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I need the assistance of a fellow genetics/bio-geek, and I thought this was the safest place for me to help find some input.

I'm trying to work out genetics for a race I'm working on in one of my sci-fi/fantasy worlds (coz I'm reaaaally obsessive about details like that).

I would like for this one trait (I call it Sablism, because it results in abnormally dark fur) to be only expressable in female individuals. I figured I should make it an X-linked trait, but Heterozygous (you would need one of each Chromosome to be a 'Sable', but since only females can have two X-chromosomes under normal circumstances, only females can be Sable).

I have been searching for examples of this, but the only real-life example I could find was calico cats (one X-chromosome codes for normal color, while the other codes for orange, resulting in a codominant situation.)

So in other words, are there any special genes that result in expression only through the presence of two differing genes (heterozygous), but would not show any significant difference in a homozygous individual?

Thanks much in advance, and I would appreciate any input, so long as it isn't a complete non sequitur. ^^;;

That wouldn't make any sense. Females have Barr bodies (random X inactivation) meaning that the genes from only one of their two X chromosomes are expressed in any given cell. That's what you see in calico cats; those two genes are not co-dominant, but rather some cells are expressing one and some cells are expressing the other.

I cannot, off the top of my head, think of any situation where a heterozygous genotype leads to a unique phenotype while both homozygous genotypes are unaffected. That doesn't mean there isn't one, but I'm a third-year bio major and I don't know of any mechanism that would create the effect you're describing unless you make "sablism" a secondary sex characteristic.

If I'm reading this right, I think all you'd need is a protein that would unfold an appropriate section of "sable" DNA based on a female specific hormone concentration. Or one that would block "sable" expression based on male hormone concentration. Then you could put either gene, sable and unfolding, any old place you want. You could get a male sable, but it wouldn't be expressed.

The only thing I can think of is if the fur color is a trait or gene that is linked and activated by only a female sex chemical. Keep in mind that this means the females with the sable gene cannot express it until puberty or when more of the female sex hormone is released.

Also, transgender male to female persons of that race only have to take the female sex hormone (and if they have the trait they will then become sable). But I don't think you have to worry about that if it is a very prmitive race without knowledge of hormones.

The only other thing I can think of is make this race different from the XY system. Make your own sex chromosome system so this trait can be expressed only in females.

That would be a secondary sex characteristic. That might be the most plausible way to go, although most such traits don't involve a stark constrast between those who have it and those who don't (eg in human females, breasts are always present but they may be different sizes; in human males facial hair is present but some guys may have more, etc)


Oh, that could work. There are actually several different sex chromosome systems in existence, and at least one of them includes the possibility of a chromosome unique to females. I can't believe I'd forgotten about that.

I considered estrogen triggering the increase in melanin production; I actually like the idea of the traits showing a while after birth, such as during puberty (which happens kind of early for this race). It also allows for a stronger allegory to the historical travails of a certain population with which we are all familiar. >.> However, my co-admin, who made up the race, as well as the idea of Sables, is more pleased with it being evident in birth and beyond.

This race is based from foxes and cats, which use the XY chromosome system, hence why I'm balking at switching around the chromosomes.

I like the idea of jmjelde, though! 8o I read that an article that says that in order for a fetus to develop as male in the uterus, a certain hormone must be present. What if the presence of this hormone prevented that protein from expressing?

Your trait could be epigenetically controlled based on any number of factors; for example, testosterone could supress the expression of the melanism gene as you suggested. If you wanted to make it more rare, other factors could lead to the suppression of the gene as well.

It is difficult to envision a scenario in a classic xy species in which the melanism is only heterozygous, unless the homozygous expression of the trait is lethal in utero (ie, bb = heterozygous wild-type, bB = melanistic, and BB = lethal).

You will have to have some combination of suppression by testosterone and homozygous lethality in order for a sex-linked, heterozygous menalism, since in xy species all of the males are automatically heterozygous wrt. their sex chromosomes.