Monday is going to be tricky

Our student body is fairly liberal and open-minded, but I still have to address a somewhat fraught topic in genetics tomorrow. We’ll be talking about sex determination, and this is a subject in which the science is clear, but also contrary to the conventional wisdom among non-scientists. I’ll be starting with the early 20th century idea that sex was entirely chromosomal and binary and work them up to the modern understanding that it’s bimodal, but non-binary, and a heck of a lot more complex than a single chromosome throwing a switch. I’m either going to get some pushback from more conservative students (which I will welcome!), or everyone is going to just shrug and tell me they already knew that, boomer.

Also, may I say that I really detest this explanation that I see all over the internet?

That’s also wrong. Sex varies on more than a single dimension, and we ought not to lump everyone with a variation from the stereotypical category as “intersex”. A lot of the older sources and some of the newer ones seem to be fond of calling everything that doesn’t fit their narrow binary “abnormal” or “deviant”.

Now I have to explain all that in a one hour lecture on the genetics of sex. Wheee.

OK, back to fussing over this lecture. That’s my day, that and putting together a summary of this week’s lab.


  1. rietpluim says

    Re: that bimodal distribution. What does the scale on the x-axis mean? Is -4 extremely masculine and +4 extremely feminine? How do they measure that?

  2. says

    Gosh I wonder what Joe Rogan and Jordan Peterson think about that topic.

    Just kidding. The drunk guy at the end of the bar probably has more valuable information. Well, actually, probably not much more valuable. Unfortunately Rogan and Peterson will share their “knowledge” widely, and that’s probably where bar-end guy gets his information to begin with.

  3. PaulBC says

    I think an underlying and extremely common misconception is the idea that “men have male genes” and “women have female genes”. In reality (and I may be bungling this in my own way), we all carry a full set of genes for either male or female phenotype, and it’s the endocrine system that determines phenotype, beginning very early in fetal development.

    The Y chromosome carries very few genes but triggers the development of testes, and I’m not sure if testosterone does the rest or if Y has some additional functions. Males have an X with the same X genes as females. Females have two X chromosomes but (and I only learned this myself a few years ago) one of them is inactivated randomly in every cell except egg cells very early in development.

    So in fact, there’s nearly complete parity on “which genes men and women have”. The issue is which are expressed in phenotype. Once you’re past early development, this regulated (I think?) almost entirely by endocrine function and not genes.

    In Swyer Syndrome, an XY zygote fails to produce testes and results in a phenotypically normal baby girl, and any differences may not be noticed until puberty. There is an elevated risk of germ cell tumors, and non-functioning ovaries (which may still appear normal). The only definite outcome post-puberty is sterility, which can have other causes. We’ve always had XY women, and nobody would have considered them other than women historically. (Note that this fits into “binary” classification, which I’m not advocating in general, just that it applies in this case.)

  4. PaulBC says

    drickard@3 Heh, that is a good point. Maybe PZ can tell his students he gave the lecture on Monday. Why didn’t they show up for it?

  5. Pierce R. Butler says

    PaulBC @ # 4: .. we all carry a full set of genes for either male or female phenotype…

    IANAGeneticist or anything close to it, but I have the impression that applies only to (XY) males and the rarer “anomalous” (XXY, XYY, etc) individuals: those XX people somehow get along without any of the “masculine” toolkit at all!

  6. calgor says

    Remember seeing sex determination discussed in a UK A-Level class on biology (17-18 year olds). The teacher brought out a chart used by medical staff training identifying various human sex identifiers and what the recommended (at the time) way forward was, either surgery, pharmaceutical or psychological. The chart only had 23 of the most common sexual determined categories listed.

  7. =8)-DX says

    @rietplum #1
    I mean theoretically it your “score” would be a collection of various sex characteristics within the current human -4 would be the tallest strongest most muscular man with the largest genitals and gonads, thickest body hair growing from the most places and largest Adam’s apple, similarly body fat distribution and skeletal shape, most testosterone, etc., you get the picture.

    Which is not how any of this works and why it’s a bad oversimplification, but people in general understand men and women tend to be more or less feminine even within the traditional binary so that chart makes a kind of layperson’s sense I guess?

    There’s also a genital classification by size chart I’ve seen that uses the same kind of distribution, where it’s literally just length.

  8. PaulBC says

    Pierce R. Butler@6 I admit, I’m not sure how much is actually on the Y chromosome that regulates anything past early development. When I look it up, I see it has 50-60 genes. That’s not much, and surely nothing like “complete instructions for constructing your male bits.”

    While I know that engineering analogies are very suspect, SRY (the “male determining gene”) reminds me of a jumper on a circuit board. The board itself has a superset of functions, and the jumper selects which one it actually does. (Caveat: in this case the “board” is just the zygote, and unlike a circuit board, it differentiates into a multicellular form that has many more functions than it started with.)

    In some animals, sex is not determined by genes at all. E.g., in some turtles, it is determined by the temperature at which their eggs incubate. So in that case, it is undeniable that male and females carry a full set of genes for both sexes.

  9. blf says

    rietpluim@1, For that matter, what does frequency on the Y-axis mean, and what are the units? Hz? If your vibrate(at? emit / absorb at?) 0.2 whatevers you’re most likely “female” or “male”? (The mildly deranged penguin says it means you haven’t had your booster, since you’re no longer in the 5G range.)

  10. Artor says

    Of course, all of this XY business only applies to humans and a few other mammals. A lot of other creature’s genders are determined by environmental factors, like the temperature their eggs were incubated at, or some other detail.

  11. charley says

    I would like to see that lecture, or another with similar information. I watched a SciShow YouTube video called There Are More Than Two Human Sexes that seems to provide a superficial overview.

  12. anat says

    Pierce R. Butler @7: Typical AFAB people do make some testosterone, in the adrenal glands, and do respond to it – hence the industry of hair-removal products marketed to women (as well as the possibility for FTM hormonal transition). So I wouldn’t say XX people lack the entire masculine toolkit.

  13. kome says

    As little as I’ve looked into this broader question of sex determination, I find sex determination in honeybees especially interesting and illustrative about how nature has never been under any obligation to conform to the all-too-human desire to have things nice and neat.

  14. Pierce R. Butler says

    PaulBC #@ # 10: … I’m not sure how much is actually on the Y chromosome that regulates anything past early development.

    The Y is relatively tiny: not at all like an X with one branch missing. I read somewhere that one of those Y-chromosome genes specifically causes hair to grow within the ears in older men (to which apparently-XX persons typically respond, “Eww.”) – so that’s at least one post-early development effect (for which no survival or reproductive advantages seem to accrue – more research needed!).

    anat @ # 16: Typical AFAB people do make some testosterone, in the adrenal glands, and do respond to it – hence the industry of hair-removal products marketed to women…

    So the one part of “the masculine toolkit” y’all have, (many of) you don’t even want. One more strike against the “intelligent design” hypothesis.

  15. PaulBC says

    How about hair growing on your earlobes? That’s a trait I carry, though I don’t flaunt it like Radhakant Bajpai. Usually I just let it blend into the rest of my hair, but I shave it after a hair cut.

  16. Ed Peters says

    Dr. Myers, if you post the lecture I would love to watch it. I last sat in a sex ed lecture in college 45 years ago and time has erased much.

  17. lumipuna says

    What does the scale on the x-axis mean? Is -4 extremely masculine and +4 extremely feminine? How do they measure that?

    I figure it should be all male because the chart is based on XY axes, not XX axes.

  18. =8)-DX says

    @Pierce R. Butler #7, #16 @anat
    Since XX SRY gene phenotypically male people exist (and incongruities in masculinisation seem to be about differential X-chromosome activation, not lack of genes for stuff per se), most of the “masculine toolkit” is definitely there. Namely everything needed to make genitals, gonads and external sex characteristics. And even these intersex people can and do use HRT to bring their body more in line with their gender.

  19. KG says

    Thanks for that reference, kenmiller@22. Just a couple of days ago, one of the Guardian’s “gender critical” columnists made the absurd claim that “all living creatures are either male or female”. Of course the non-human world offers plenty of straightforward counterexamples, but it’s good to have a single, highly authoritative refutation with regard to our own species!

  20. Howard Brazee says

    It’s important to remind people that labels lie, or at least are misleading.
    It is so comfortable to believe we understand things by putting labels on them. But the world is not that simple, and in particular, biology isn’t that simple.

  21. says

    KG @ #27:

    Just a couple of days ago, one of the Guardian’s “gender critical” columnists made the absurd claim that “all living creatures are either male or female”.

    How does something like that get published in the Guardian? Is there no fact-checking for columnists at all?

  22. says

    I disagree about the graph. Sure, it is a very very crude oversimplification, but every explanation is an oversimplification and this graf is informative and accurate enough to be a good starting point for explaining the problematics to a layman. Which means most people, even progressives.

  23. Pierce R. Butler says

    =8)-DX @ # 26 – Thanks for bringing that up – it’s led me on an educational tangent.


    … Usually, it is caused by unequal crossing-over between X and Y chromosomes during meiosis in the father, which results in the X chromosome containing the normally-male SRY gene. When this X combines with a normal X from the mother during fertilization, the result is an XX male.

    so the “masculine toolkit” is there (at least as far as I understand it), just on a different shelf.

  24. blf says

    SC@29, That — all living creatures are either male or female — is arguably taken out of context. The Grauniad’s journalist, Hadley Freeman, was interviewing Marget Atwood, Playing With Fire: Margaret Atwood on feminism, culture wars and speaking her mind: “I’m very willing to listen, but not to be scammed”, is attempting to explain a question to Ms Atwood:

    “OK, let me say this again,” [Atwood] says more sharply. “This is going to take a while to settle down, but XY and XX are not the only chromosomal combinations possible. Look it up, OK? This has been in flux for a very long time and in the Bible, a male wearing female clothes would be —” and she makes a slicing gesture across her neck. “You want to do that? No.”

    Gender-critical people would argue that those are different issues, I say.

    “What is a gender-critical person?” she asks

    It’s someone who believes that all living creatures are either male or female and that rare chromosomal variations don’t disprove that.

    “I’m not going to argue about this. That’s not what my book is about and that’s not what we’re here to discuss,” she says.

    As I read the above excerpt, Ms Freeman is answering Ms Atwood’s question, much to the annoyance of Ms Atwood. At least in the above excerpt, there’s nothing to indicate Ms Freeman’s opinions or beliefs.

    My current understanding is “gender-critical person” is a codephrase for TERF, i.e., a transphobe hiding behind a distorted idea of feminism.

    Elsewhere in the interview, you get the sense Ms Atwood is not a TERF, but she has apparently made contradictory statements, etc. As a competent journalist, Ms Freeman seemed to be trying to understand Ms Atwood’s position / beliefs better, which ultimately led to the above-excerpted exchange.

    My take on the whole interview is neither Ms Atwood nor Ms Freeman is being very clear on this particular subject, and using this interview as the only source, it’s close-to tealeaf-reading to definitely claim either or both are, or are not, TERFs.

    (Disclaimer: I’ve generally liked Ms Freeman’s work in the Grauniad over the years. I’ve never read anything by Ms Atwood that I liked, and that includes what they are perhaps best known for, The Handmaid’s Tale.)

  25. PaulBC says


    … rare chromosomal variations don’t disprove that.

    I have what I think is a consistent take on this that has served me well since I started to give it any thought.

    Clusters are real things that can be observed and quantified statistically. Disjoint categories are, for the most part, a human invention that saves us the effort of thinking too hard about clusters. They are useful the same way drawing stick figures may be a useful way to depict people doing stuff. But arguing about categories is a case of mistaking the map for the territory.

    “Male” and “female” are clearly very useful clusters of sexual identity. But to dismiss everything else as “rare chromosomal variations” shows not merely a reluctance to accept complexity, but that this kind of complexity is so abhorrent as to be ruled out. Taken to its extreme, it’s a form of essentialism.

  26. blf says

    @35, Yes. So what do either interviewing journalist Hadley Freeman or interviewed author Marget Atwood think / believe ?

  27. says

    There are primate troops in the 100s to 1000s. 0.2-2.0% isn’t insignificant if they have a disproportionate impact somewhere. Lots of people around me reacting to intense words I seem to have feel for. I still think the “F-word” is suggestive of our collective inability to deal with sex/intercourse even if it’s other things too.

    I think the “faster than controls at naming manipulated items” bit might relate to a “T-rex motion based vision” joke.
    “Speeded processing of grammar and tool knowledge in Tourette’s syndrome”

  28. says

    That should say “…I have little feel for…” in my 37 but even that is not quite right. The “less feel for” is due to an intensity increase so it’s like a category is a higher volume making interaction more difficult.

  29. PaulBC says

    Brony, Social Justice Cenobite@37

    There are primate troops in the 100s to 1000s. 0.2-2.0% isn’t insignificant if they have a disproportionate impact somewhere.

    I’d personally place the emphasis differently (not trying to persuade you; that’s your business). The absolute number of people globally with “rare chromosomal variations” is large enough to fill a city. It’s an outrage to dismiss their experience as anything other than fully human and fully legitimate whether or not they have any “impact.” These are all people with the right to a fulfilling life, whether their fulfillment lies in reducing or accepting their variation. (Of course that’s true if there were only one of them, but I admit the sheer number helps me feel the urgency.)

    It’s bizarre to me that anyone would ever attempt to dismiss someone’s concerns as irrelevant merely because they are not shared by a great enough number. It’s Procrustean: “You don’t fit my preconceptions, so either conform or get out of my sight.” I would prefer to treat people as individuals.

  30. says

    Maybe I wasn’t explicit enough. I have the tourette syndrome. I’m describing an experience of having a disproportionate impact I’m in the process of figuring out. But that doesn’t mean there isn’t a better way of expressing it even if that isn’t exactly the same as a chromosomal difference.

  31. PaulBC says

    Brony, Social Justice Cenobite@40 Let me see if I can make point clear. I think I have one.

    What I’ve noticed over the years (let’s say going back to the 80s when I started paying attention) is a large part of public discourse, notably LGBTQ-rights, going to great efforts to legitimate a type of human experience as “normal” as if this is really the point. The point, it seems to me, is that each individual has some path to fulfillment in life, and provided they can follow it without harming others, they are entitled to do so. If there is some issue of societal harm, it needs to be mitigated, but that’s a pragmatic issue, not a justification of one experience over another.

    I may have missed your point about primates, but I thought it was that even a 0.2% outlier population could have positive impact and therefore a purpose to the community. Thus, their existence is “justified” despite being what was once called deviant or abnormal. So a cartoon evolutionary psychologist might try to explain the persistence of rare traits in this way. I.e. that they serve some adaptive function to the community.

    Of course, high achievers are also < 0.2% outliers by some metric and in principle we accept the need for these outliers (Olympic runners, concert pianists, Nobel laureates, etc). There is just a tendency to separate the “good” from “bad” outliers.

    My point is just that I don’t care. People are all different and if they’re not doing harm, their percentage in the population has absolutely no bearing on their consideration in ethical terms. Be kind and accept others on their own terms if they are not doing harm.

  32. says

    It’s not about justifying. It just is. I have no negative feelings about fuck. It’s about other people’s feelings to me otherwise it’s synonymous for the act of sex.

    I’m not saying this with a lot of negativity myself. You do care, you feel some negativity about another way (more than one actually) of categorizing experience. Me putting myself and tourette syndrome into a 100s to 1000s of primates frame is similar to that chromosomal variation frame. I don’t see this as incompatible.

  33. PaulBC says

    Brony, Social Justice Cenobite@42 To be clear, I did miss your point completely, assuming it was about the impact of Tourette Syndrome rather than a general statement about 0.2% outliers.

    Yeah, I see how that has impact (words have a disproportionate effect). I don’t really have anything to add to what you wrote on that. There’s an impact to others that needs to be mitigated in some reasonable way. It’s a tougher issue than people failing to conform to traditional sexual roles, which can be solved easily by ignoring tradition.

  34. PaulBC says

    Brony, Social Justice Cenobite@44 It’s not the same as your experience, but I have found my whole life that I often express myself in ways that I consider reasonable but others find in poor taste or simply impertinent. It doesn’t really matter who’s “right.” I just adapt to societal expectations to a degree as a pragmatic gesture.

  35. says

    @PaulBC 45
    This is where “this is how it works” might come into play. I was being descriptive. Other people can feel however they want about that. I still choose what I do with fuckity fuck. Just seeing the distinction between anatomy and action insults is useful.

    I can still choose to do want with the word, what stops me is…

  36. says

    KG @ #27:

    Thanks for that reference, kenmiller@22.

    Seconding this. From that article:

    Halfway through her fifth decade and pregnant with her third child, the woman learned for the first time that a large part of her body was chromosomally male. “That’s kind of science-fiction material for someone who just came in for an amniocentesis,” says James.

    Many people never discover their condition unless they seek help for infertility, or discover it through some other brush with medicine. Last year, for example, surgeons reported that they had been operating on a hernia in a man, when they discovered that he had a womb. The man was 70, and had fathered four children.


    Simone de Beauvoir in The Second Sex in 1949 (this is from the 2011 translation by Constance Borde and Sheila Malovany-Chevallier; and from far from the most progressive chapter in the book) was on the right track:

    For both sexes, fertilization and the beginning of embryonic development occur in an identical way; the epithelial tissue destined to evolve into a gonad is undifferentiated at the outset; at a certain stage of maturation testicles take shape or later the ovary takes form. This explains why there are many intermediaries between hermaphroditism and gonochorism; very often one of the sexes possesses certain organs characteristic of the complementary sex: the toad is the most striking case of that; the male has an atrophied ovary called Bidder’s organ that can be made to produce eggs artificially…. Even in species where sexual division is the most clear-cut, there are individuals that are both male and female simultaneously: cases of intersexuality are numerous in animals and human beings; and in butterflies and crustaceans there are examples of gynandromorphism in which male and female characteristics are juxtaposed in a kind of mosaic….

    For vertebrates, it is essentially the gonadic hormones that play a regulatory role. Many experiments have demonstrated that varying the endocrine milieu makes it possible to act on sex discrimination; other grafting and castration experiments carried out on adult animals [grr] have led to the modern theory of sexuality: in male and female vertebrates, the soma is identical and can be considered a neutral element; the action of the gonad gives it its sexual characteristics; some of the secreted hormones act as stimulants and others as inhibitors; the genital tract itself is somatic, and embryology shows that it takes shape under the influence of hormones from bisexual precursors….

    Interestingly, the book’s first English translator (1952), Howard M. Parshley, made a point to append a footnote:

    In connection with this view, it must be remembered that that in man and many animals the soma is not strictly neutral, since all its cells are genotypically either male (XY) or female (XX). This is why the young individual normally produces either the male or the female hormonal environment, leading normally to the development of either male or female characteristics.

    Here’s a fascinating 2010 article about Parshley’s views and his translation of TSS.

  37. says

    blf @ #34, thanks for the information. Sadly, Freeman is a transphobe.

    At one point she suggests:

    [Chimamanda Ngozi] Adichie describes this performative rush towards public proof of purity as “American feminism”, and I’ve been thinking a lot about American puritanism recently. The US was originally colonised by the pilgrims, a group of people who believed Europe was too louche for their pure Calvinist ways, and so headed west. America thinks of itself as a very free country, and in many ways it is, but it has always maintained this strong strand of purer-than-thou-ism.

    It’s funny because I happen to be reading a book about a Puritan who was literally a judge in the Salem witch trials and who, were he alive today, would probably not be a transphobe like Freeman. For TERFs to paint themselves rather than trans people as the victims of a puritanical impulse is pathetic.

  38. blf says

    SC@34, Thanks for the link! Yes, sadly, it does seem Grauniad journalist Hadley Freeman is, or is very close to being, a TERF. The relevant excerpt:

    I know some people think I’m on the wrong side of history because I believe my gender is a feeling and my biology is a fact. This is known as a gender-critical belief and it is protected under the [UK’s] Equality Act. Nonetheless, I’ve lost at least a dozen friends over this — mainly from the US, but also in the UK, friends who have told me my beliefs are transphobic, even when I tell them that I support everyone’s right to live the way they want. […]

    The first sentence smells strongly of transphobia, as some of their ex-friends have apparently told them. The remaining sentences are wibbly-wobbly, either trying to justify their belief (the spurious reference to the Equality Act suggests they are quite defensive (presumably about being told they seem to be transphobic)), but they conclude at the end of the excerpt on level sensible ground. So another possible reading (of perhaps several) is they don’t understand the science at even a superficial level, but realise being uncomfortable with “sex ≠ gender” is not a valid reason to treat transitioned people any differently.

    Having said that, I presume Ms Freeman would be very unhappy with an individual who transitioned to female using the lady’s toilet — a more practical test, maybe, of whether or not someone is a TERF (or “gender-critical”) than trying to parse their writings?

    Anyways, thanks for that relatively recent link (June 2021) giving some insight into Ms Freeman’s beliefs on the subject.

  39. Kevin Karplus says

    Perhaps it would be best to start with an animal that has non-chomosomal sex determination—like crocodilians, whose sex is determined by the temperature of the eggs during a critical week of development.