Chromosomes, biological sex and gender

Trans activists are forcefully exploiting people’s weak knowledge of chromosomes & hormones. Even the Government and the NHS (!) are coming out with unscientific claptrap – which could be mildly amusing if they weren’t about to make life-changing decisions based on utter nonsense.

We’ve never really needed to know about genetics unless we were directly affected by an abnormality – but we can’t have ignorance leading a seismic change to our society, so let’s clue ourselves up.

It’s a bit like the deep sea – we know it’s there and roughly what’s in it, but we leave the details to the experts. If we decide to take up oceanography we’re suddenly playing catch-up while everyone who once watched an underwater video tries to tell us they know all about it. We can’t afford to let ‘amateur experts’ guide us now. This post shows you where to find genuine expert information, and summarises the main points.

It won’t be a full genetics & endocrinology course in a couple of blog posts! But I can cover the bases, which will make you better informed than 90% of the population. In her excellent post on intersex & dimorphism, Catherine Drury tackled the identifarian hypothesis – that one’s internal gender feeling dictates one’s physical sex. I’m going to explore the nuts & bolts of physical sex: chromosomes & hormones. Some of these do suggest a possible ‘gender feeling’ – read on.

Quick links in this post: sex chromosomes | XX-XY sexual reproduction | females & malesmultiple anomalies | turner syndrome | triple X | klinefelter XXY | jacobs XYY | trisomy 21 | trisomy 16 | third sex | mosaicismgenes | sry | genital anomalies | conclusions

Sex chromosomes

There are only two sex chromosomes, X and Y

Here they are. They don’t really look much like the letters X and Y, but the photographer with the microscope has helpfully coloured them pink & blue. I guess stereotyping has its uses.

Chromosomes & Hormones, Biological sex -
Y chromosome (left) and X (right)

Human sexes

There are only two sexes, XX and XY.

In all mammals, an ovum (egg; females are born with a lifetime supply) contains one sex chromosome, an X. Sperm (male bodies make them as required) also contain a single sex chromosome. With each ejaculation of approx 300 million sperm, about half contain a Y and the other half contain an X chromosome.

Chromosomes & Hormones, Biological sex -
Sperm, meet egg (animation by Michelle Davis)

Both sperm and egg also contain 22 other chromosomes, which code for all of your DNA. When these join up, they will form the 23 chromosome pairs that make you what you are: unique (unless you’re a monozygotic twin.)

Here’s a good video animation of fertilisation and fusion (5 minutes, has audio description.) The fused cell is called a zygote.

Only females produce ova (eggs). Only males produce sperm.

How do we know for sure? Because that is the definition of female and male.

Mammals, insects, birds, fish, most reptiles and many plants reproduce sexually. The names for the egg & sperm producing varieties are female and male respectively. It doesn’t make any sense to say these words mean something different in humans, because humans are mammals. And like all the other mammals, this is how we make new ones!

You need an XY male + an XX female to make a baby.

So. A woman’s ovum, with its single X chromosome, accepts one of a man’s spermatozoa. This sperm has either a single Y or X chromosome. When egg & sperm fuse, the two sets of chromosomes pair up into either XX or XY.

If it turns out to be XX, the resultant foetus will be female and (all being well) will be born with a set of her own eggs.

XY codes for a male foetus, which will be born with the equipment to manufacture his own sperm when the time comes.

The presence or absence of a Y chromosome determines whether the foetus will be male or female. Anyone who has a Y is genetically male, even with irregular sexual development.

It isn’t true that all babies start out female.

The sex is determined as soon as a zygote is formed. Sex organs develop later, but this development follows the coding set by the XX or XY chromosome pair.

You have 30,000,000,000,000 sex chromosome pairs.

More than 30 trillion cells in your body contain XX or XY chromosomes. Even your spit and toenails are female or male. Going off at a slight tangent, you can’t have a male brain in a female body because the brain cells of a female body are female.

It doesn’t always work perfectly.

It’s nature, right? Nature constantly experiments, throwing out mutations and anomalies that may or may not prove beneficial to the species. Some mutations continue because they advantage the species if not the ‘mutant’ individual – more on this later. Others keep happening because nothing’s perfect and, if a few anomalies don’t destroy the species, nature doesn’t care.

Anomalies during fertilisation & fusion

There are millions of genetic abnormalities. They can be caused by heredity, illness, environmental factors or plain old luck. After miscarriage, the most common reproductive anomaly is twins.


If an ovary releases more than one egg by mistake, all the mature ova can be fertilised by different sperm (one sperm each.) Each fused zygote then goes on to develop as an individual with unique DNA. They’ll be fraternal twins/triplets/etc.

If a single ovum fuses with a sperm but splits afterwards, two or more zygotes can then go on to form separate babies. They have identical DNA, because only a single fusion happened in the first place. These are identical twins/triplets. One in 250 births (well, two in one) is monozygotic twins.

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Chromosome number irregularities

A human should have 46 chromosomes forming 23 pairs, but some don’t. The most prevalent aneuploidy (number error) is Down syndrome – the next four most widespread relate to the sex chromosomes, accounting for about 30 million people.
Autosomes: chromsosomes 1-22
Allosomes: 23, the sex chromosomes

  • Monosomy X

Turner Syndrome. The second sex chromosome is missing or broken, leaving only a single X. There is no Monosomy Y because the X chromosome carries functions necessary to life. The only viable monosomy, Turner Syndrome accounts for at least 10% of all miscarriages.

Girls born with Turner’s have developmental issues, which can be dealt with these days although incomplete sex organs mean they won’t be able to get pregnant normally; some can use IVF. Turner’s women are likely to have a shorter than average life span.

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  • Trisomy XXX

Most girls with Triple X Syndrome go undiagnosed and live normal, healthy lives. Like the other sex-chromosome trisomies, XXX tends to confer added height. Some developmental delays and skeletal problems may be present.

Triple X syndrome - Chromosomes & Hormones, Biological sex -

  • Trisomy XXY 

Klinefelter Syndrome, the second most prevalent trisomy after Down Syndrome. This is one of the very few genetic abnormalities that could be called a gender identity disorder – but that would be a stretch. It’s the most common sex chromosome disorder in humans, and millions of XXY men are quite happy with their Y chromosomes.

Boys with Klinefelter’s tend to grow taller than their siblings. They may have developmental delays, some female-type characteristics, and suffer many health difficulties which can be treated. They have incomplete sexual development and can’t reproduce normally, but doctors are often able to extract viable sperm.

Many XXY males go undiagnosed until they seek treatment for symptoms. A few are more comfortable with a ‘sex change’ to live as women. These people may also have a gene expression issue, which we’re coming to later.

Chromosomes & Hormones, Biological sex -
Matt Roseworne, happy XXY man, vlogs about his condition
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Adele Markham, genetically XXY male, eventually decided to pass as female
  • Trisomy XYY

Jacobs Syndrome. XYY boys grow taller than their siblings. There may be some developmental delays or learning difficulties, but in general this abnormality causes no problems. Like Triple-X, it mostly goes unnoticed.

  • Trisomy 21

Down Syndrome. Caused by an extra copy of Chromosome 21, Down’s is the only fully viable autosomal trisomy. It brings developmental and health problems, which can be managed. Fertility is often impaired but between 30% – 50% of people with Trisomy 21 are able to have children.

Trisomy 21 - Chromosomes & Hormones, Biological sex -
Gigi Cunningham has an extra Chromosome 21
Trisomy 21 - Chromosomes & Hormones, Biological sex -
Polly Gibson and Joe Minogue have 6 copies of Chromosome 21 between them.
  • Trisomy 16

This abnormality is incompatible with life. It’s the second most common chromosomal cause of miscarriage after Turner syndrome. If only some of the cells have an extra Chromosome 16, the foetus may survive – often with serious defects.

  • Each of the above occurs in about 1/1,000 pregnancies.

Other, less frequent trisomies of the autosomal chromosomes (1-22) include Edwards Syndrome and Patau Syndrome. Survival rates are extremely poor.

  • Tetrasomies +

Further sex-chromosome aneuploidies (wrong numbers) are much rarer, extended versions of those described above. Wikipedia links: Tetrasomy XXXX | XXYY Syndrome | Pentasomy XXXXX | XXXXY. These abnormalities come with severe health issues and infertility, although some quadruple-X women get away without symptoms.

Autosomal tetrasomies that are compatible with life are very rare, and not extra copies of a full chromosome but of some of the ‘arms’: information.

For other chromosomal abnormalities, see here and this list.

– The most common genetic defects in live babies are:

  • Heart defects: More than a million births worldwide yearly.
  • Neural tube defects (including spina bifida): Nearly 324,000 births worldwide yearly.
  • Blood disorders (such as sickle cell disease and thalassemia): More than 307,000 births worldwide yearly.
  • Down syndrome: More than 217,000 births worldwide yearly.
  • G6PD deficiency (enzyme deficiency that causes anaemia): More than 177,000 births worldwide yearly.

X/Y number errors are not extra sexes

Chromosomes & Hormones, Biological sex - FairPlayForWomen.comThis assertion deserves an award for childishness. It seems to be based on the idea that male or female can only be XY or XX, and must have a fully-working reproductive system.

Let’s just ignore the fact that the same people also say chromosomes & reproduction have nothing to do with sex. I’m taking one argument at a time here.

Thus, they say, people with unusual sex-chromosome numbers and/or faulty reproduction aren’t male or female. They’re something else – other sexes! That’s pretty offensive to the billions of women & men who don’t fit the ‘perfect’ model. And the fact remains that there are only two sexes.

A third sex would have to boast a different method of reproduction.

No mammals have a third sex; we either make babies in the usual way (XX+XY) or we can’t. If we can’t, modern medicine often helps – but that isn’t a new means of reproduction, it’s a doctor facilitating the egg & sperm process.

Having incomplete, unformed or oddly-shaped reproductive organs doesn’t mean you’re neither male nor female. It’s a genetic defect like, say, a cleft palate. We don’t insist that people born with cleft palate are a different kind of human! A male who develops breasts is still a man. I’m not knocking Adele Markham’s personal choice, but 9 million men have Klinefelter’s. Markham is far from typical.

Mosaicism & chimerism

Chromosomes & Hormones, Biological sex -

Some of our cells can be genetically different from the others. It’s thought that 70% of us have some sort of mosaicism. It generally isn’t very interesting, but can be huge – with some parts of the body having a genetic disorder and some not. I once knew a lovely woman who had a fatal mosaic disorder; she died in her early twenties, even though half of her body was still healthy. People can have mosaic sexual development and even mosaic sex, where some of their body is male and some female. This doesn’t make them “hermaphrodites”, as they are not sexually both female & male. Geneticists sometimes use the term hermaphrodite but it is, sensibly, falling out of favour.

Chimerism is similar, but caused by cells from two individuals merging in utero. It usually indicates that the remaining individual started as a twin. Again, this can result in a mixture of male & female cells, and/or a mixture of chromosomally normal & abnormal tissues. There’s also something called microchimerism, where cells from each pregnancy remain in a woman’s body and then make their way into subsequent foetuses. This could mean that all younger siblings contain a few cells of their elders.

Significant mosaic & chimeric developments are rare; significant sexual mix-ups even rarer. While they’re intriguing, they’re certainly not a reason to suppose biological sex doesn’t exist or there are more than two sexes. Some sexually chimeric/mosaic women have given birth – this in itself proves that they are women, despite their unusual cell patterns.

Finally … all women are mosaic, chromosomally speaking! This is to do with the fact that we can’t live with both X chromosomes active, so half of our Xs are deactivated – long read here, and a great video below by Veritasium, “Why Women Are Stripey”…


Each human cell has 46 molecules of double-stranded DNA, one strand from the egg & one from the sperm. Tightly wound up, these molecules are your chromosomes. People with disorders like those above have 45 or 47 instead of 46.  Genes are segments of DNA. Each chunk contains instructions for making proteins & enzymes.

Book analogy: Chromosomes are the story, genes are sentences.
Computing analogy: Chromosomes are the program, genes are code blocks.

Chromosomes & Hormones, Biological sex - FairPlayForWomen.comBy combining in different ways, the proteins built from our genes make up all the different parts of our bodies. As far as we currently know, we have about 20,000 protein-coding genes and a heck of a lot more DNA segments with unknown function. There’s plenty of scope for small errors here – and small errors can have big consequences. To complicate matters a bit more, genes work by making copies of themselves called messenger RNA – this then goes off into the body to pass on the instructions. Very amazing but, as we all know, making loads of copies can lead to degradation or mistakes.

Geneticists don’t yet know everything about genes, and this post’s already getting long. You might like these articles: What are genes | How can genes go wrong? | Human genome

The SRY gene

Also called testis-determining factor, this gene is part of the Y chromosome. It prevents a foetus developing female pattern reproductive organs and codes for male ones.

If SRY is defective the failure can lead to Swyer Syndrome, where an XY male develops female genitalia with a uterus and fallopian tubes, but only has streak gonads; these can easily become cancerous. Swyer’s is normally treated with female hormones after puberty should have occurred; a handful of people with this condition can gestate an implanted foetus. I’m not sure this could be called a ‘gender identity’ disorder: people with Swyer’s are generally in no doubt that they are female despite their Y chromosome. Prevalence unknown, but fewer than 1/20,000.

A faulty SRY can sometimes cause 46, XY disorders of sex development. This is characterised by ambiguous and/or poorly formed genitalia, rarely with a uterus and fallopian tubes. Some children with this disorder are surgically ‘corrected’ unhelpfully, and there’s a high risk of cancer in the affected organs.

SRY can turn up on the X chromosome of a genetic female. This causes 46, XX testicular disorder of sex development. The child’s born with male-looking genitalia although these may be poorly formed. They are typically raised as boys despite their XX chromosomes, and treated with male hormones after puberty should have occurred. Most have a masculine identity; all are infertile.

Ovotesticular disorder of sex development describes the existence of both ovarian and testicular tissue (in malformed gonads, not a full set of both). Usually accompanied by poorly-formed and often indeterminate genitals, this rare condition has caused gender distress in patients who were surgically ‘corrected’ as babies.


Genital anomalies

By no means all “genital anomalies” are problematic. There are very uncommon things like double uterus and double penis as well as fairly common ones like micropenis and mild hypospadias.

Many people seem to have the idea that anything out of the norm – or perhaps they mean their stereotyped ideal – is a sign of failed sexual development. And that disorderly sexual development means there is some ground between ‘woman’ and ‘man’ where magical beings may reside, determining their sex by power of thought and reproducing by unknown means.

Maybe I’ve been around the block a few more times, but I’m vividly aware that people come in all shapes & sizes. They are still functioning human beings – no need to redefine the species just because we don’t all match.

In case you’re curious (you are, aren’t you?), a man with diphallia and a woman with didelphys had a Reddit thread.

What I’ve learned

  1. About 220,000 babies are born with Down Syndrome each year and 130,000 with Klinefelter.
  2. Abnormal sexual development doesn’t mean there are more than two human sexes.
  3. It can’t mean this, because a third sex would have to be able to reproduce another way.
  4. If humans could reproduce without being sexually dimorphic, we wouldn’t be mammals.
  5. Some chromosome irregularities lead to incomplete or unusual sex development.
  6. One such condition – Klinefelter’s – may cause identity issues, but this is rare.
  7. Problematic SRY genes can cause people to develop reproductive organs associated with the opposite sex. Their identity usually matches their bodily appearance, they receive medical help, and they’re infertile.

Reading about SRY-related disorders has left me feeling very upset about the adults who are ‘transing’ children as I write. They’re using medicine to mimic serious genetic disorders in healthy children. As adults they’ll be infertile; their undeveloped gonads will presumably be cancer-prone as all unusable organs are; they will actually need more surgery and prolonged medication than people with Swyer’s or 46, XX tdsd.

NHS, you should be deeply ashamed.

Coming soon: the relationships between sex chromosomes, genes and hormones.

9 thoughts on “Chromosomes, biological sex and gender

  • 2017.08.12 at 23:01

    Great reading.. Thank you.

  • 2017.08.13 at 13:50

    I’m hoping you’ll address what we’ve learned from gender identity development in intersex. That at least some of it is rooted in biology rather than entirely learned. I’m thinking specifically of Reiner’s work with Cloacal Exstrophy. My condition’s another good example. I have a Y chromosome in some cells but not in others. The asymmetric form of my condition is called Mixed Gonadal Dysgenesis. Most are born with one good testis, one streak ovary, ambiguous genitals toward the feminine end of the scale, and a partial uterus. Although standard practice has been to castrate and raise as girls, quite a few would consider themselves boys. I have the symmetric form–two ovatestes, no uterus, and genitals toward the masculine end of the scale. Quite a few of us consider ourselves girls.

    If you define male and female based strictly on the ability to procreate, then some of us don’t fit into either category. I would also point out that your definition isn’t used in any legal system on the planet. Most of the time M or F gets written on a birth certificate based on a glance between the legs. Certainly not on whether or not the baby will be fertile in one direction or the other.

    My second novel, A Proper Young Lady, is about a woman with Partial Androgen Insensitivity Syndrome who fathers a child with a little help from a clinic. They extract sperm precursors from gonadal biopsies. That possibility was suggested by the head of UCLA’s genetics department.

    Anyway, thanks for the great article. Most of it. 😉

    • 2017.08.13 at 18:29

      At least some of it may well be rooted in biology – that’s one of the things I’m exploring. Gender identity, however, is not a quantifiable or even a definable thing. Would you say women with Swyer Syndrome have a female gender identity, or are they simply ‘female’ because that’s how their body looks and how they were raised?

      If the latter, then you’d be saying Swyer women are “cis” although chromosomally male. If you put it down to trans identity, how do you tie that in with their naturally female-pattern body type?

      I haven’t defined male & female according to fertility. I explicitly said that was a naive point of view. For what it’s worth, I couldn’t produce live babies and am confident of my sex! Apart from the trans lobby and certain fundamental religions, everyone understands biological sex as a clear binary with predictable features which, being biological, are sometimes imperfect.

      “Sometimes imperfect” means that:

      • In a high proportion of people, some aspect of their sexual/reproductive presentation will diverge from what’s expected – like mine.
      • In a low proportion, several aspects may diverge – as in Klinefelter.
      • In a tiny proportion, sexual & reproductive features may be jumbled so the person isn’t easily classified as woman or man: in such cases – perhaps like yours – a longer description may be needed and the person should certainly be allowed to choose their label if medical considerations allow.

      All this is WELL within the range of natural variation. The simple fact is, anything outside the normal range results in unviable life. Even frequent variations fail often (eg Turner Syndrome) or always (Trisomy 16) – the ‘grey’ area is narrow.

      It’s ridiculous to claim that natural binaries aren’t binary if there is any gradation. This is not how life works. We know what night & day mean; dusk & dawn don’t invalidate the binary. Life & death aren’t even fully distinct but we all know what you mean if you say someone died. Should they be in a vegetative state with minimal brain activity, that will require a longer explanation.

      My aim in these posts is to explore the normal range of human variation because people are getting terribly confused about what is a really basic, and quite simple matter: biological sex.

  • 2017.08.13 at 22:20

    Mapping the diversity of biology to categories is fairly arbitrary. Reducing it to what’s between the ears or what’s between the legs erases other important distinctions. Where you draw the lines between male, female, and intersex is political for quite a few people; they want the lines drawn in particular places based on their own goals. Historically, there’s been an effort to erase intersex through secrecy, shame, hormones, and cosmetic surgeries. There’s also been an effort to erase intersex by re-classifying some conditions as non-intersex. The question one should be asking is why do people want to define intersex in a way that minimizes the conditions included? Or that maximizes the conditions included? Why not leave the scientific definition as it is? Intersex is a mix of male and female biological characteristics. Sure there’s a threshold beyond which it matters. Some conditions result in medical issues. Some in ambiguous genitals. But again, arguing over which specific conditions are included and how serious one must be to be included tends to be based on politics rather than science.

    My original birth certificate said male. That was the doctor’s call. He reduced my biology to a strict binary based on the shape of what was between my legs. And frankly, they were much more worried about my health issues than my legal sex. I’m intersex. The law allowed only male and female. So they put male. But then the sex designation on a birth certificate has to do with legal sex rather than biological sex. Most of the time those two are the same, but there’s a real distinction.

    The genetics that made me intersex gave me a feminine face and made me tiny and frail as a child. I have spatial deficits and had hypermobile joints, so I couldn’t play sports. Like most girls, I had to be aware of how dangerous boys could be. I played with girls because they were safer and because my parents’ friends had only daughters and because all but one of my cousins was female. Mom taught me to cook and sew and clean. Bought me an EasyBake oven. I borrowed my mom’s or my sister’s clothes on a pretty regular basis. But I was at least nominally supposed to be a boy. So my childhood socialization was atypical. Did I have a strong sense of being a girl? No, though I wanted to be a mom and have babies. At times I wanted to be a boy–tall and strong and agile like my brothers. What girl doesn’t? But I knew I wasn’t a boy.

    At 22 I looked like a flat-chested girl with the privates of a little boy. When I turned down testosterone, the doctor said I wouldn’t have any trouble being accepted as a girl. He was right. The bullying stopped as soon as I told people I was a girl rather than a boy. Because I fit their expectations better. That was 1974. No one has questioned my sex or my gender since then.

    For better or worse, society treats me as a woman. My adult socialization is typical of a woman with a career in a misogynous high tech industry. Genderwise, I consider myself a fairly ordinary woman. I’m legally female. If you saw me naked, you’d find that I’m pretty stereotypically female in height and build. My husband would assure you that I’m capable of vaginal intercourse as a female. But my biology’s intersex rather than strictly female. And my intersex biology doesn’t keep me from being female. But that’s because my intersex biology maps to a category that is considered female socially/legally.

    • 2017.08.16 at 23:29

      I think it’s crucial to realise, Lianne, that you are an exception. The proportion of people born with “intersex” conditions is tiny. Many of you have been mistreated both medically and socially. That is a shame and it’s truly horrible that it still goes on (not so much in the UK any more, thank goodness.)

      What I feel to be very dangerous for our society is this extrapolation, from a very few occurrences of abnormal genetic developments, to sweeping statements about biological sex and – even more egregiously – gender, which has no empirical foundation. Sex is actually a binary in mammals: a biologically necessary binary. Gender’s as binary as you want to make it, but it’s nothing more than a set of rules & restrictions created by mankind.

  • 2017.08.27 at 17:19

    As an Intersex person, this is great read and I can understand why the Trans wants to reduce and erase Intersex people like myself.

    • 2017.08.28 at 15:24

      Thanks, Nicky. It’s good to know I’ve helped explain how intersex conditions are useful to the trans agenda. I’m sorry they’re using people like yourself. It’s not as if you haven’t got enough to cope with! People with DSDs are still fighting for the recognition & respect they deserve – the last thing you need is a larger, better-funded movement confusing the issues even further.

      • 2017.08.30 at 02:05

        It’s why being Intersex/DSD is hard enough when I have to deal with my medical issue. The last thing I need is the Trans trying to erase my medical condition.


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