X linked gene but more common in females?

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  • #1
I am writing a genetics guide for my fictional cats. I know that most of it isn't actual cat genetics but it is still genetics.

There is 1 particular set of pattern genes that is more commonly expressed in females than in males, the lion pattern.

I haven't decided yet whether it should be dominant or recessive but since it is rare, I think it should be recessive.

I figured since males rarely have this that it should be X linked. But then again, X linked usually means more common in males or equal precedence in both sexes, not more common in females. So how could this be?

First off, there is the pattern itself. This is age dependent, not sex dependent. So X linked dominant would work for this.

But the pattern itself changes with age. The kittens are born with rosettes and simple spots, just like this lion cub:


As they grow older the rosettes disappear but there are still simple spots:


And finally, here is the sex dependent part of this lion pattern. Once they are fully grown, the spots either completely disappear like in most cases or uncommonly, stay there, primarily on the belly and chest. Out of these cases of spot retention, most of them are females. Males rarely retain their spots but there are a few cases of spot retention in males.


Here, you can see spots on the chest of this very pale lioness, no rosettes, just simple spots.


And here is a picture of a very pale male lion that has spots. Spots in male lions are rarer than spots in lionesses.

But my question is, how could this spot retention be X linked and more common in females?

The only two ways that I can think of an X linked gene being more common in females is that:

1) An X linked gene is connected to another X linked gene and that other X linked gene is connected to an autosomal gene. Females have double the gene expression and thus are more likely to have spot retention.


2) A Y linked gene produces a protein that inhibits spot retention in males. But in the very rare case that this Y linked gene itself is inhibited, the males have spot retention.

But is there anything else besides a chain of genes that could make something that is X linked more common in females instead of in males?

Answers and Replies

  • #2
Cute mini-lions!

Genetically, recessive alleles (versions of a particular gene) will not show a phenotype more often in females, which I think you understand.
Since males have only a single X-chromosome, any male that gets a single copy of the allele you are discussing (which I will call spot) will have it's phenotype. This would reflect it frequency in the breeding population.
Females will have to have two copies of spot to show it's phenotype which will be a less frequent occurrence. This will be it's (frequency)2 in the breeding population, less likely.

If it were a dominant allele, a single copy would produce a phenotype anytime either a male or female gets a copy of the allele. Since the female gets two shots at getting a spot allele, that possibility will be slightly higher. Something like 2 x frequency in the breeding population.

1) An X linked gene is connected to another X linked gene and that other X linked gene is connected to an autosomal gene. Females have double the gene expression and thus are more likely to have spot retention.
A gene that is X-linked is on the X-chromosome. It can therefore not be also linked to (connected in your quote) an autosomal gene (which would be on another chromosome). Autosomal chromosomes are any chromosome other than the X or Y-chromosomes.

A more likely explanation is that the spot phenotype is in some way affected by the sex of the individual animal. This would be like like secondary sex characteristics, like men growing a beard or female mammary development. Mostly has to do with the hormones in an individual and the sex they have developed as which would affect their sensitivities to the hormones. Depending on the details, this might still leave room for some of the variability you described.
  • #3

Melanophores,chromatophores are skin structures that affect skin pigmentation. And in some cases follicular "spots". They are a series of alleles of the same genetic code, dominant and recessive. So, for more spots you need more dominant alleles.

I am taking your question to be hypothetical mostly, right? So you need to have multiple loci on the X chromosome. So with 2 X chromosomes you find more intense skin color effects.

There is one problem with the whole idea. Lyonization. (not joking) [PLAIN]https://en.wikipedia.org/wiki/Barr_body[/PLAIN] [Broken]

In mammals only one X chromosome exists and is expressed in the nucleus. The other one becomes an inactivated blob called a Barr body that resides outside of the nucleus.

So putting genes on the X chromosome and expecting to get 'double' effects in females has problems. The 2 X effect does not exist. Because in mammals, two active X chromosomes in any diploid female tissue at one time does not exist.
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  • #4
Another possibility is 'dosage effect'. That is a characteristic that for full expression needs to doses of the allele. One dose gives a weak effect, but more visible or more expressed when the overall coat colour is pale?

We probably haven't exhausted the possibilities.

Er - I printed out the question, these imaginary cats are done very well, then came back and answered. I felt a bit uneasy because I have heard of gene dosage, but also of X inactivation. Then I saw Jim McNamara's post.

However doesn't the inactivation take place at certain developmental stages? So that you can have mosaicism – a given gene being expressed in half the cells. Unfortunately I do not well remember and am not very up-to-date. Could you explain by supposing the spotted phenotype in females to be the result of non-expression are of a certain gene in half the cells? That it is just the mosaicism that causes the spots? That it would be a rare in males because either expression of the allele in question in all the cells or non-expression in all the cells gives rise to the uniform colour?
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  • #5
Lyonization appears to happen in early embryonic stages. Whichever of the two chromosomes is 'evicted' stays evicted in most of the tissue for the life of the animal. Chimeras notwithstanding. You could look at lyonization as creating a kind of weak chimera I guess. Especially if there are massive differences between lots of the loci on the X chromosomes.


Some secondary mammalian characteristics reside somatic chromosomes, and are switched on/off by other activity mediated by the X chromosomes. Kind of like a cascade which epenguin mentions. So maybe we could have a 'switch' related to sexual dimorphism, switches do exist.

I'm trying to use some non-technical terms, which sometimes helps. Lots of definitions can clutter semantic meaning - what you are really discussing. Worse might be undefined technical terms.
  • #6
The male appears to lose those spots as they age because their coats and manes become darker, longer, and thicker than the females on account of testosterone; also, a thicker, longer coat will conceal and blend in those spots. "Males with darker manes were hotter than those with lighter manes," The same thing is also found in humans and some other animals: even in a human population composed of just one "race", females find males with darker skin shades hotter, and males find females with lighter skin shades hotter. And within that "race", it is indeed found that men are usually a shade or so darker. Here, the lionesses are retaining both spots and a lighter color, while the lions grow into having a slightly darker color, made to look even darker by a longer and thicker coat/mane, concealing the spots. No need to invoke a genetic explanation- it's hormones! Lions are probably stinkier and dirtier than the females, so that probably keeps them appearing darker and less spotty, too!

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