If you’re multicellular, you can’t help but be mosaic

I quite liked this article by Emily Willingham on the male/female brain: she points out something that is obviously true, that individual brains are a complicated mosaic of traits, and that you simply can’t reduce all of the variety to a simple binary.

Humans want tidy patterns, to have things link up neatly and make sense. Our brains strain to make these connections whether they are genuine or not. What’s more difficult is looking past illusory patterns and thinking more deeply about what we’re really seeing. As tempting as it is to collapse a human’s entire being, including the brain, into a single term – male, female – an honest look at how we really behave makes such reductionism look shallow, at best.

The most observant among us manage this in-depth examination. These acute observers are not the scientists, who can be remarkably myopic and rigid within their corners of research, but the storytellers. You can’t tell a good story about people if you’re not a keen observer of human behaviour, and it’s in our storytelling traditions that we find example after example of an inherent if unconscious understanding of the mosaic brain.

It was good, but the article didn’t go in the direction I expected it to go — I guess I’m more reductionist than I thought. When I started reading about brains being a mosaic of different properties, I first leapt to the idea of epigenetic variability in the regulation of of “male” and “female” genes. (Isn’t that where you go, too?)

Here’s the deal. You know that there is this beautifully intricate process called X-chromosome inactivation, or dosage compensation, in which individuals with more than one X chromosome epigenetically shut down most of the genes on all the additional X chromosomes. It’s a really cool process — think about it, the molecules involved have to count chromosomes, and I don’t understand how they do that — but it’s also leaky. About 15% of the genes on the X chromosome escape inactivation, by unclear mechanisms. And further, some of those genes are variable in how frequently they escape inactivation.

For a given gene, escape from X inactivation is not necessarily consistent between individuals or between tissues and/or cells within an individual. A comprehensive survey in human confirms the original observation that some genes only escape X inactivation in subsets of cells. Interestingly, many genes (∼10% of X-linked genes) behave in this manner, resulting in potentially variable expression levels between female tissues and individuals. Whether, in turn, this generates female phenotypic variation is an interesting possibility that remains to be explored. Partial or variable escape from X inactivation is in agreement with progressive incorporation of genes into the X up-regulation/X inactivation systems once the Y paralog degenerated.

Female brains are literally mosaic in their patterns of gene expression — some cells will have one X chromosome active, others will have the other X chromosome switched on, and further, there is a random pattern of genes on the X chromosome that are variably silenced, and different patches of the brain will use different alleles.

And guys, don’t think you can escape this phenomenon: epigenetic regulation is simply a little bit sloppy, and so your brains have random inactivation of some undetermined set of regulated alleles. It’s not as simple as having a boy set of genes and a girl set of genes that are uniformly and universally working in a predictable way in every brain.

But that’s only adding to Willingham’s points. Male and female are clearly insufficient labels to pigeonhole the complexity of the human brain.

By the way, if you want to see the inverse of this argument, take a look at this inane tweet.

Among sexually-reproducing multicellular organisms, nearly every species has two distinct gamete types (“anisogamy”).

Female: big, cytoplasmically rich, sessile.
Male: small and mobile.

That is true. If only we could reduce human beings to single reproductive cells, the gender binary would be valid. Unfortunately for their perspective, it isn’t. Our brains are not single-celled gametes, and I would hope don’t even contain any gametes, which would be creepy and icky.


  1. Owlmirror says

    I’ve been pondering the possibility that at least some trans people might be the result of genetic mosaicism; opposite-sex twins absorbed during development. But I don’t know how valid an idea that is, let alone whether it’s at all helpful.

    The OP is probably a better point: all brains are mosaics.

    Biology is messy.

  2. says

    @1 Owlmirror

    (NB: I am not a biologist, PZ please correct me if I go off the rails here.)

    As I understand it, a developing human fetus will default to female, but if the SRY gene is present (occasionally NOT on the Y chromosome, just to be extra fun) it triggers several cascades of masculinizing hormones. In the most common cases, all these cascades are either present (result: cis male) or absent (result: cis female), but because biology is always messy this doesn’t invariably happen. so if you get some of them, and not others, you wind up with certain kinds of intersex conditions based on the fact that some developmental stages were influenced by androgens and some were not. AFAIK this is the current leading theory for the biological origin of trans people – if one’s body develops one way and brain the opposite, you’ll get a binary trans person.

    This is a very high-level view that as described doesn’t directly account for the existence of nonbinary people, so presumably there’s more to it – but taking it as serviceable for the phenomenon it is trying to describe (à la the Bohr model of hydrogen), it would make being transgender a type of intersex condition, and not require any more unusual phenomenon than a glitchy hormone distribution to do it.

  3. monad says

    I know this isn’t the point, but I’m not sure that even passes a simplistic view of sexual reproduction. Taking all multicellular animals, then saying gamete type -> sex -> gender except in rare cases? I suppose animals that don’t even have different sexes like earthworms and garden snails and C. elegans are “rare”, in that they are much less common than insects, but there are still a lot more than mammals. No idea what sex -> gender in most animals would mean.

  4. chris61 says

    Emily Willingham wrote:

    And our brains, rather than fitting neatly into some binary of being ‘male’ or ‘female’, might also be mosaics, quilted together from pieces with varying hues of ‘masculine’ and ‘feminine’ expression.

    I believe the technical term for that statement is codswallop.

  5. Azkyroth, B*Cos[F(u)]==Y says

    I believe the technical term for that statement is codswallop.


  6. chris61 says

    @9 Azkyroth
    Biologically speaking it’s nonsense. Brains aren’t binary like gametes are but neither are they mosaics of ‘masculine’ and ‘feminine’ expression any more than a liver or a heart or an adrenal gland is.

  7. Allison says

    Re: “male brain”, “female brain”, and trans people.

    I note a distinct lack of skepticism in even discussing these concepts. It’s hardly proven that there even exists differences between women’s and men’s brains that would allow one to detect a person’s sex from a brain scan, other than perhaps sensory regions for organs that exist in only one sex. So far, all I have heard is that someone took a group of (cis?) men and women, did brain scans of some kind, and looked for any differences. Not to mention that biological sex itself is more complicated than the simple binary model.

    As for transness being caused by a male brain in a female body or vice versa, I have never heard any research into whether there are detectable differences in brains that would predict (let alone account for) a person being trans. There’s also the problem of deciding whether a given person is trans or not for the purposes of such a study. “Cis” and “trans” are binary concepts, but gender identity, gender performance, etc., are anything but binary, and people don’t even agree on the terms.

    I’m also really skeptical of the male/female brain ideas because they are what I would expect people to latch onto. Trans people are looking for “objective” proof that transness is real and not just some psychotic delusion. And both cis and trans people are raised to see sex and gender in binary terms, so gendered brains are what they expect to see. And one of the most common errors that both scientists and lay people make is to see what they expect to see, not what’s actually out there.

  8. Phil Crawford says

    Here is my attempt at trans 101, flat-earth worldview is useful in most circumstances but far from a complete description of the world. This is akin to a simple binary description of gender. Euclidean and heliocentric descriptions of the world explain a much broader range of phenomena and are like a gender-spectrum description of what it means to be trans. There is a small but growing population who recognizes their identity as outside of the M-F binary. Non-binary describes a wide variety of genders and is like quantum mechanics in that it is the most fine-grained view but takes a lot of explanation and few people really understand it.