Jul 09, 2020 — by Natural Selections , in Canton, NY
“Tortoise shell” coloring in a (usually female) cat is the result of it being a genetic mosaic. The gene for fur coloring is being expressed separately by both the X-chromosomes in different parts of the cat’s body. Photo: Chriss Haight Pagani, Creative Commons, some rights reserved
Jul 09, 2020 — In most cases, we get half our genes from one parent, half from the other. But it doesn’t always happen that way. Parts of the genetic inheritance can be turned on or off, and genes from other familial sources can play a role in shaping the individual body. Martha Foley and Curt Stager explore what happens when there are ripples in the gene pool.
Martha Foley: Let’s talk about mosaics and chimera, not in the art world or in mythology, but in, maybe even in our own bodies. So you need to explain to me, what is a mosaic? What do you mean when you say that?
Curt Stager: If you refer to that in terms of a person, then you are talking about genetics, and genes. So a mosaic would be, you know, different colored tiles all mixed together to make a picture. And you have different cells in your body that make the picture of you. And each cell has its’ genes in it, well in our case, everyone of your cells actually has two sets of genes in it.
MF: Mother and father.
CS: Right, one from mother one from father. So it turns out your cells really don’t need to use both copies all of the time, and sometimes they will shut one set off, and it’s sort of random.
MF: So like cell by cell, not like your whole body?
CS: It could do that sometimes, like maybe mismatched eyes or something, but usually it’s pretty random. And so it would be hard to even notice it unless you were a geneticist checking, you know? So this cell says, I’m going to use my moms’ gene and dads’ gene, and usually you won’t notice that kind of thing.
MF: That’s what makes us who we are.
CS: Yeah, so one classic example in humans would be with women, you’d have two of these X chromosomes, as they’re called, and you’re genetically different from a guy who would have an X and a Y chromosome. If you have two Xs’ as a woman, then in each of your cells you’d say, “well I only need one; I’ll turn one of them off.” And so a woman’s cells are a mix of which one is using which. So normally you don’t notice it, but it turns out with cats you can actually see the effect of that, because the fur color genes are on that X chromosome. So with these female tortoise-shell cats they look sort of like a calico that is all, you know, finely broken up lines with some blacks in there.
MF: Oh yeah, it’s a jumble.
CS: More of a jumble right? And that makes sense because it’s sort of random, you know, this cell over here made a little piece of fur that’s orange, and this one said, “Nope, I want to be black, right next to it.” So it used moms’ and dads’ different genes, and so the tortoise-shell cat is a visible mosaic, whereas with a human you wouldn’t really notice it so much.
MF: Well interesting. Side question here, is that why there are no male tortoise-shell cats? Or are there male tortoise-shell cats? You know, that thing about there are no male calicos?
CS: There actually are a few, but when you check the males out, they actually have an extra X chromosome. Which is a whole other thing, and they can actually turn them on and off.
MF: So, now on to chimera. What is that?
CS: Yeah that seams sort of similar, but it’s actually different. With the mosaics, the genes all actually came from the original embryo, from the original mom and dad. With a chimera you’ve got four parents, or more. It’s because there were two separate embryos that got together, and it could be from, in the case of humans it could’ve been exchanging cells in the womb with your siblings, or having a few of moms’ cells blending in or vise-versa.
MF: So you are saying that even outside of having twins, there are cells from other siblings still in the moms’ body?
CS: Yeah, inside there you can have connections like, actually with other primates, like marmosets, they’re actually all chimeras because when they are in the womb the cord blood mingles between all of the siblings, so they have these embryonic stem cells that turn into body parts. They’re blends of each other when they’re adults.
MF: But it’s just not that common in humans?
CS: Well we’re not totally sure because it’s hard to tell, and especially if it’s small cells, and people actually have found it in unfortunate circumstances, in legal custody battles when a woman would have a genetic test, maybe for various reasons, maybe a tissue transplant or something, and then it turns out her genes don’t seem to match exactly the kid, and it’s because of this mixing and matching. If they had taken the sample from another part of her body, you know she might have had genes from some sibling or something inside her, and it confused the whole test.
MF: Wow, that’s kind of crazy.
CS: You could say, you know, if you have an organ transplant, you know like a pig heart valve, you’re a chimera too, in a way. We see that in a positive light in that case
MF: Wow, kind of a lot to think about here, mosaics and chimera.