Genetic inheritance of the brain

I raised this this issue in this thread but the point didn’t get answered, so I thought I’d post it as its own question.

In this article the author claims that all the genes for the male brain are inherited from one’s mother and whereas female brains have an equal mixture of inheritance from the mother and father. This, he claims, is because males only have one X chromosome, and all that.

Now, for this to be true, it would mean that brain development depends only on the X chromosome, yes? Is this in fact true?

The author seems to be unaware that humans have 23 chromosome pairs, and that only one pair out of those 23 is sex-linked. For this theory to be true all of a human’s brain development would need to depend on the X chromosome and not in any of other 22 chromosome pairs at all. I find that very unlikely.

Nope. This is where you got it wrong. The author claims half the XX chromosomes come from the mother and the other half from the father. That’s correct.

That’s different from saying ALL gene expression is from the X chromosome. Yes, MANY brain related genes come from the X chromosome, but not ALL the genes.

It’s not expressed quite so clearly that everybody would get the distinction, but the author didn’t say anything technically inaccurate.

The author speaks of calico cats, saying that the expression of genes requires one of the chromosomes to be switched off. Never heard that before. Has a mechanism been discovered or proposed for that? Is it supposed to be the case for all the matched pairs?

X-inactivation. Curious that marsupials only inactivate paternal X chromosomes, as opposed to the random pattern in placental mammals.

The article doesn’t actually make ANY claims about how this mosaicism affects male or female brains. The entire thing just explains X-inactivation and its consequences in humans (and cats). The relevant parts below:

This is a description of an imaginary, hypothetical scan that shows nothing more than which X chromosome is active. It has nothing whatsoever to do with any actual brain function.

If it’s still unclear, the author ends with this:

In other words, he says outright that the things you’re claiming he says in this article are actually going to be addressed in the NEXT article.

And yes, for the record, of course only those genes carried on the X chromosome will be affected by X-chromosome inactivation. The 22 pairs of autosomes are totally independent of it.

I also want to point out that this isn’t new information. It was in high school textbooks in the early 90s. Also, different animals handle the X dosage problem differently. Fruit flies do it by doubling gene expression on the single X in males, rather than reducing expression in females like we do.

Thank you for the clarification!

What’s really cool is that some girls are carriers for a x-linked gene that causes colorblindness. Females have one X inactivated and which of the two X chromosomes gets inactivated is random. The X being inactive is passed down to cells that are produced when that cell divides during growth and development. In some of those girls, patches of their retinas are “color blind” and some normal depending on which X got inactivated- the one with normal gene for color vision or the one with colorblindness causing gene.

On the microlevel, no different than calico cats coat coloration.

The article doesn’t make that claim. If you go back and read it, he says “some genes”, not “all genes”. In fact, he should have made it clear that he’s talking about “very few genes”.

So we should guess that he will say “random X-inactivation is why women are scatterbrained”.

By the way, the OP talks about “genetic inheritance” and equates that to X-inactivation. Just to be clear, what the article is talking about has nothing at all to do with inheritance of genes. Each and every cell in any woman’s body has exactly the same DNA*. X-inactivation affects only which copies of that information are read. It’s like having one copy of a book on the shelf, and the other copy open in front of you. You’re only reading one copy, but you still have both copies.

And while I’m at it, it may also be worth pointing out that the “inactive” X chromosome is not actually completely inactive. If it were, then Turner syndrome patients - women born with only one X chromosome - would be completely physically normal. They’re not.

*“Exact” in the biological sense, where it means “except for a few pages’ worth of exceptions that are really interesting, but not worth getting into here”

Does X-inactivation pertain to sexual dimorphism? It would seem that having a reduced genetic expression might account for females being smaller, having more compact features, or lacking some kinds of decoration.

X inactivation serves to equalize gene expression. Males are XY, females XX. Without inactivating one female X, males would have half as much gene expression compared to females.

Some organisms, like fruit flies, accomplish normalization by amplifying expression off the male single X and don’t inactivate any of the female Xs.

Generalizations, of course. More complicated in real life.

Not really. Those are regular old adaptations. After all, some species have bigger females, while others have smaller females. These traits will be sex-influenced, for sure (for instance, female hormones downregulating growth genes), but not necessarily sex-linked (ie, on the X chromosome).

Well, a little more than half, since males also have the Y, which (normal) females lack entirely. But the Y carries very few genes, so it’s not much more than half.

X-linked genes are the ones being balanced. The imbalance between Y and no Y is clearly not as critical. Even if the body didn’t compensate for the difference in number of X-chromosomes, there would still be the Y/no Y difference. Again, it’s not that simple, but close enough!

The article linked in the OP is three years old. Here is the subsequent entry (note that where he says he will explore more next time, that is a jump link).

His assertion is that female mosaicism dilutes genetic traits by spreading them out. Men have one X, so it gets expressed, while women’s two Xs get expressed more or less randomly, which evens out undesirable traits instead of presenting them. He says the bell curves for men, in things like IQ, are inverse mirrors of those for women: there are more really smart and really stupid men, with the average falling at the minimum, whereas the really smart and really stupid women are in the minority, with the average at the maximum. There is a little bit more, and a link to the next article.

Wow, what a [del]steaming pile of bullshit[/del] interesting but highly speculative hypothesis. For there to be anything to the idea that genetic mosaicism decreases variability in female brain function, by increasing genetic variability in the brain, the following would have to be true:

  1. There would have to be significant X-inactivation mosaicism in the brain. X-inactivation happens pretty early in development, which results in very large patches of identical cells. For the calico cat that’s pretty obvious: on the epidermis there are usually a handful to a few dozen different patches. Does X-inactivation happen before or after brain cell precursors originate? That could mean the difference between a single “patch” of genetically identical neurons, vs possibly a very finely divided brain with many different “patches” of X-inactivation.

  2. There would have to be a substantial number of genes involved in brain development and function on the X chromosome.

Now both of those may be true, but the author of those fluffy pop-psychology articles does not bother to state the evidence for either. Without such evidence it’s just another convenient rationalization and just-so story.

Yup. What he said ^

It might charitably be called a mildly interesting thought experiment, but there’s no evidence for it whatsoever, and it doesn’t really hold up to much scrutiny. I, for one, find it highly unlikely that something as complex as human cognition is so influenced by a single gene (or a small number of genes) that a significant proportion of variability can be mapped to a single chromosome.

Large patches is what I expected from looking at the Calico cat image in the article. But that doesn’t square with IvoryTowerDenizen’s post that

With large patches, the two eyes might be different from each other, but I wouldn’t expect multiple patches in a single retina. Do you (or anyone) have a source for the typical size of this X-chromosome inactivation mosaicism in humans?