by Katherine J. Wu
figures by Daniel Utter

Let’s talk about sex.

Seriously. Not intercourse, though – more about how genetic sex is programmed during development. Sexual identity has been in the news often lately, and unsurprisingly so: the past few years have yielded sweeping reforms in civil rights, spurring new conflicts surrounding everything from age-old battles in gender equality to legislation enforcing anti-transgender bathrooms. It’s a complicated subject, to say the least. With regards to science, we don’t know enough about gender identity to draw any conclusions about its biological underpinnings, and certainly not about what is “right” or “wrong.” We are only now beginning to fully understand how mammalian sexual identity has evolved, and its dependence on the sex determination systems that allow biological development of sexual characteristics in different organisms.

The sex determination we’ll discuss today is (unfortunately?) not the dogged resolve to copulate. Most multicellular organisms, humans included, use sexual reproduction to reproduce. Compared to asexual reproduction, in which cells can simply create carbon copies of themselves, sexual reproduction allows for the introduction of genetic diversity into a population. In most sexually reproducing organisms, there are two sexes – but the ways in which these sexes are determined and the ways in which they manifest vary greatly. What are the ways in which sexual characteristics are encoded? Why are there so many systems for one seemingly common result?

SRY not SRY

We were all taught the classic recipe in grade school: an X chromosome from mom and an X chromosome from dad will yield a genetic female, while an X chromosome from mom and a Y chromosome from dad will yield a genetic male. The XY sex determination system (Figure 1A) is certainly what’s most familiar to us, and it’s used in most other mammals, as well as a few select insects and plants. Briefly, human cells all carry chromosomes, which carry our genes. When egg meets sperm, each parent contributes 22 non-sex chromosomes and one sex chromosome – always an X from the mother, and either an X or Y from the father. Thus, the contribution from the father determines the sex of the baby[1].

Following fertilization, a fetus begins to develop. At first, its sexual organs manifest as a genderless gonad, or sex gland – basically a small, thick ridge of tissue near what will become the abdomen. The “default” sex (i.e., without any other further input) is actually female – however, the presence of a gene called SRY on the Y chromosome initiates the release of testosterone and the formation of male sex organs. SRY is a transcription factor – a genetic element that can turn on the expression of other genes. In this way, SRY is like the master switch to turn on the suite of “male” genes in a developing organism. Thus, the presence of a single Y chromosome switches on the male pathway, something that is clear in what’s called Klinefelter Syndrome, in which individuals carry two X chromosomes and one Y chromosome, but develop testes and appear generally “male.” Without the presence of a Y chromosome, and thus without SRY, cells secrete estrogen instead of testosterone, and an XX baby develops female sexual organs.

It seems like a pretty clear system – but it wouldn’t be biology without exceptions and extra rules muddying the waters. When it comes to sex chromosomes, X’s and Y’s are not the only ingredients available. Many other sex determination systems exist, and the concept of “male” vs. “female” isn’t quite as simple as humans once thought.

The Birds and the Bees (and Some Other Things Too)

Unsurprisingly, with the immense variation observed in our natural world, more than one sex determination system exists. Ours, XY, is not even predominant. A few key examples tend to predominate: the ZW system in birds, XO in insects, haplodiploidy, and environmental sex determination systems.

The ZW system (Figure 1B) exists in birds and some reptiles, and operates opposite of XY: females get the mixed set of sex chromosomes (ZW), while males are ZZ. Thus, unlike in humans, the mother’s contribution determines the sex of the progeny[2]. Just as the mammalian Y chromosome carries the male-determining SRY, the avian W chromosome carries similar master switches FET1 and ASW, which are necessary for female development of the offspring, which will otherwise “default” to male.

In the XO sex determination system (Figure 1C), which is found in several insects, females are still XX, but instead of carrying a Y chromosome, males simply carry a single X – the “O” in “XO” indicates the absence of a second sex chromosome. Each sperm carries either an X chromosome or no sex chromosome at all – but once again, as in XY, the father’s contribution determines the sex of the offspring.

Figure 1: Five (of many) sex determination systems. A. XY system In humans, females are XX and males are XY. B. ZW system In birds, females are ZW and males are ZZ. C. XO system In insects, females have two sex chromosomes, but males have only one sex chromosome (while retaining two copies of all non-sex chromosomes). D. Haplodiploidy In honeybees, females again have two sex chromosomes while males have one, but in this case, males have only one copy of every chromosome. E. Thermal regulation In some reptiles, the temperature of the surrounding environment determines the sex of the offspring.
Figure 1: Five (of many) sex determination systems. A. XY system In humans, females are XX and males are XY. B. ZW system In birds, females are ZW and males are ZZ. C. XO system In insects, females have two sex chromosomes, but males have only one sex chromosome (while retaining two copies of all non-sex chromosomes). D. Haplodiploidy In honeybees, females again have two sex chromosomes while males have one, but in this case, males have only one copy of every chromosome. E. Thermal regulation In some reptiles, the temperature of the surrounding environment determines the sex of the offspring.

After this, things start to get a little weirder. Honeybees utilize the system of haplodiploidy (Figure 1D), in which unfertilized eggs (which carry only one set of chromosomes and are thus haploid) develop into males and fertilized eggs (which carry two sets of chromosomes and are thus diploid) develop into females. Importantly, this is distinct from the XO system, where progeny inherit two copies of all non-sex chromosomes, regardless of sex; in haplodiploidy, males inherit only one copy of all chromosomes, sex and non-sex (Figure 2A).

Honeybee colonies typically center around a single fertile queen, serviced by an army of male drones and female workers. The queen lays a vast number of eggs, some of which are fertilized and develop into females. Those that remain unfertilized develop into males. Thus, in this system, males have no fathers and can produce no sons. Furthermore, if a queen chooses only one drone to mate with, all her daughters will share 75% of their genes with each other (unlike in humans, where siblings share 50% of their genes) because they each inherit the full set of their father’s genes, rather than just half. While this system seems vastly overcomplicated, it is believed to have been evolved to promote the social nature of honeybees: as a female worker, it turns out to be more evolutionarily advantageous to protect your sisters (with whom you share 75% of your genes) than it is to produce daughters of your own (with whom you share only 50% of your genes) (Figure 2B). Thus, the community structure revolves around the queen. This is an interesting case where the genetically determined sex of individuals shapes their role within the larger community.

Figure 2: Sex determination in honeybees. A. Honeybee haplodiploidy Fertilized eggs inherit a set of chromosomes from their mother and a set of chromosomes from their father, and are always female. Unfertilized eggs receive half their mother’s chromosomes and are always male; males have no fathers. B. Sisters before mothers Each daughter receives all her father’s chromosomes and half her mother’s chromosomes. Thus, sisters are more related to one another (75%) than they each are to their mothers (50%).
Figure 2: Sex determination in honeybees. A. Honeybee haplodiploidy Fertilized eggs inherit a set of chromosomes from their mother and a set of chromosomes from their father, and are always female. Unfertilized eggs receive half their mother’s chromosomes and are always male; males have no fathers. B. Sisters before mothers Each daughter receives all her father’s chromosomes and half her mother’s chromosomes. Thus, sisters are more related to one another (75%) than they each are to their mothers (50%).

Finally, there exist systems in which sex determination isn’t dependent on chromosomes at all. In alligators and some turtles, the temperature at which the egg is incubated during a sensitive period determines sex: lower temperatures produce females, higher temperatures produce males (the phenomenon of “cool chicks” and “hot dudes”) (Figure 1E). However, this rule does not hold true in every species – sometimes the opposite rule is in effect, or temperatures at either extreme produce one sex, while an intermediate temperature produces the other. Some snails and fish are actually able to reverse sex midway through life, depending on environmental conditions, in a process called sex reversal. Thus, genetic sex is a far more fluid process than one might assume.

The fact that genetic sex can be directed by the flip of a single switch may be surprising. Sex is complex – but then again, there are a lot of other factors at play and, clearly, environment can have a big influence on how sex expresses itself. Additionally, there are many documented cases of humans with a genetic sex that appears “contrary” to their physical appearance. For instance, we know of genetically XX persons who have developed testes and external characteristics of men, and genetic XYs who develop as females. An example of the latter case occurs in Swyer Syndrome, often when there is a mutation in the SRY gene. While the rest of the Y chromosome is left intact, a malfunctioning SRY means that the male “switch” is never flipped, and the indifferent gonads do not get signals to become testes. Swyer Syndrome patients develop externally as female, but do not have ovaries and are infertile.

Finally, inheriting extra or too few chromosomes can considerably alter how sex manifests. Klinefelter is a common example, as well as Turner Syndrome (XO), where a sex chromosome is missing, often leading to developmental defects. Thus, all it takes is a small genetic change to turn SRY, or any of the genes it targets, on (or off).

(De)Generation Y

We know very little about how sexual reproduction and sex determination systems evolved – the theories are, of course, difficult to test. But another important question is, once sexual reproduction did evolve, why did it branch off in so many ways? And, perhaps more pressingly, is it still evolving in ways that could affect us?

The answers are still mostly elusive. There has been some indication that the XY and ZW systems are still connected to a common ancestor, even though they manifested a complete reversal somewhere down the line. One small but interesting line of evidence lies in the platypus, which encodes a whopping 10 sex chromosomes (males are XYXYXYXYXY instead of XY – apparently, size matters to platypodes) that bear great similarity to the bird Z chromosome, but technically operate under XY sex determination rules. Interestingly, though, the platypus Y lacks SRY. Thus, platypodes may end up being the “missing link” between these two systems.

Furthermore, analysis of the Y chromosome has indicated that it probably evolved from the X chromosome, acquiring some literal “man power” along the way. This “differentiation” event solidly distinguished the roles of the two chromosomes, and they began to evolve away from each other over time. In its current state, the Y chromosome is much smaller than the X chromosome, and appears to have lost the unnecessary X genes[3] along the way. Y continues to exhibit signs of this (very, very slow) Y degeneration as time progresses. In fact, the XO sex determination system is believed to have arisen from complete loss of an effective Y chromosome that was ultimately discarded for its relative inefficiency. There’s no need for panic, though, XY readers – your Y chromosome is unlikely to be going anywhere anytime soon, or maybe ever. Complete loss of Y is a pretty extreme event, and much evidence has accumulated that the loss of genes from the Y chromosome will ultimately plateau.

Plenty of Fish (Sexes) in the Sea

Sex determination in humans is fairly well established. But our system is neither the dominant mode of sex determination, nor a more “correct” version of it. A final lesson comes in with the fairly new discovery of polygenic sex determination (PSD), wherein multiple genes and chromosomes contribute to the ultimate sex of offspring. This can take the form of XY and ZW systems being combined into the same species, for instance. Domesticated cantaloupes (yes, the fruit) produce four sexes, and there is some evidence that several species of fish rely on PSD). This system is still poorly understood, but importantly, the added variation on each side of the equation indicates that even genetic sex is often not a binary trait. Perhaps it’s time to rethink our preconceptions about the divides between “male” and “female.”

Katherine Wu is a third-year graduate student in the Biological and Biomedical Sciences Program at Harvard University and is, as far as she knows, XX and not XY.

[1] … Making King Henry VIII’s decision to behead a bunch of his wives for failing to bear him sons a little misinformed. Oops. Not that he could have known at the time – no one did. So, moot point.

[2] If only Anne Boleyn had been a pigeon.

[3] For the most part, the genes on the X chromosome only need to be present in one copy, hence the favoring of “loss” of duplicates on the Y chromosome. In fact, in women, who have two X chromosomes, one X chromosome in each cell is packaged into a dormant state called a Barr body. This actually happens at random in each cell (that is, it’s not always the X from mom or the X from dad that’s turned off – it can be one or the other), resulting in “mosaicism.” This is actually how the coats of calico cats are patterned, and why the vast majority of calicos are female! Cool fact: if you stumble upon a male calico cat, it is almost certainly XXY.

Further reading

  1. To learn more about why honeybees can’t produce males from fertilized eggs, check out this brief article: http://mbe.oxfordjournals.org/content/early/2013/12/06/molbev.mst232.full
  2. An excellent review on the evolution of sex determination: http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899

Featured image from Wikimedia Commons.

33 thoughts on ““I’m XY and I Know It”: Sex Determination Systems 101

    1. Male and female He created them:

      Chromosomes determine if you are male
      or female, not your state of mind. Not surgery, and not hormones.

      Simple scientific fact.

      1. Bro did you read the article at all?

        “Additionally, there are many documented cases of humans with a genetic sex that appears “contrary” to their physical appearance. For instance, we know of genetically XX persons who have developed testes and external characteristics of men, and genetic XYs who develop as females.”

    2. agreed, it seem to me that they are just pandering to the last brain cell to appease these people

  1. On strictly medical terms. there are 3 things that are possible, the baby has vagina or a penis. or in VERY RARE cases has a different condition that due to how nature decides to shape the human body, can end differently, usually denoted as a defect. modern practices have become better at solving this issue however.
    As a surgeon, i apologize if i seem slightly absorbed, but i have to take a firm stance on this topic. currently the two most logical medical solutions to gender are:
    1. you are what you are born with, body modifications are to be treated as nothing more than cosmetic. I can somewhat agree as whether you have a penis or vagina at this current time, you still carry either XY or XX in your genes and your size/muscle growth will reflect it.
    2. you are what you are equipped with at the current time. this is rather simple. if you have a penis, you don’t go in the women’s restrooms and vice-versa. this stemmed from what seemed to be a rise in alleged cross-sex rape cases from both sexes, in public restrooms. I see the point here, if you have a little girl. you don’t want her to have an encounter with a man in the women’s restrooms.
    Both seem pretty understandable.
    but the one thing that i cannot accept is the talk of non-binary gender spectrum things.
    gender isn’t a spectrum. you have male and female. subcategories of this are male and female. if your trans, then you are still either male or female. you are not nonbinary gender fluid whatever. i’m not saying your less human, but i am saying that you are not as to who you claim to be. i guess at this point we have to go as far as to say it like this:
    look at your hand right now. it is made of cells. those cells came from DNA. which is either XY or XX. nothing more, nothing less. both sexes have their far share of problems the other cannot fully understand, neither has it easy. but moving away from your self in order to feel like your self IS IN FACT a very bad mental state to be in. sure, it works for a moment. you get lost in the illusion and feel more free. but take a moment to put the illusion aside and reflect on your life choices and their consequences.
    physicality and medicality should always be put over opinions and politics. so its like a direct slap in the face that large, reputable universities, whose job it is to provide students with this valuable knowledge have turned. they are abandoning everything they once were for nothing.
    more people know what gender studies are than the are people who know what STEM even means. that should be a pretty large indicator right there.
    the fact that this is even a thing to discuss sickens me. we could have spent this time gaining a deeper understanding of cancer, or figuring out how to purify nuclear waste, build an animal shelter, go donate blood, go plant something. imagine all the time and money this new culture has demanded that could have went to better more productive things that have a practical purpose.

    1. Looking at the first and third comentors it would appear they are experiencing the back fire effect.
      1)Our first commentator jumps to calling this junk science. Not really helpful or insightful. The point was to get people thinking about gender and how we as humans perceive it.

      2)Our third commentator speaks of medical terms, but deviates from medical to personal matters and then includes scare tactics i.e. rape. Please don’t ignore that a disturbed individual will do horrible things regardless of what sign is on a bathroom door. It was also not mentioned that same sex rape happens.

      Both the article and the surgeon miss 2 conditions that make the gender based on DNA not so simple.
      A) Intersex, a condition where a person does not fit into a nice neat column A or B of gender/sex organs. So XX is a girl and XY is a boy. Neat, easy, no thought. Just look at you hand, well what if your hand’s DNA reads XXY or XYY. Not so neat and simple right. Is check their pants they say. Ah that can be a problem too. Many who have XXY do not hconditionnizable genitals. So what to do?
      B) the second condition chimerism is when one person hold DNA of more than one more individual. So that hand may not be XX like the rest of you. Google thus one if you’ve never heard of it. The part that makes this interesting is there was no way of telling if you have this without testing multiple parts of the body’s DNA. So yes, it is possible you’re brain (or at least the part of it) could be XY and the rest if you is XX. The thing is medical science can only guess at how many people have this particular condition since it requires multiple DNA tests and I don’t see many people signing up invasive surgery to have their brain tested just because they feel different.

      The real reason this is all being discussed is that gender matters to society. If male amd female were true equals then it would not matter if you were male, female, or inbetween. We have constructed roles and expectations based upon our (perceived) gender. It’s not about whether you are male or females; it’s the notion of conforming to assigned roles. It’s not about genitalia. It’s not about not about restrooms any more than it was about fountains or where you could sit in a bus. It IS about human beings. If you have never been discriminated against because of the way you look or act you will never unstand what it is like. I don’t want people to see my skin tone or hair and expect me to be something just because of my genetics. Treating people a certain way just because of their genetics is not a good idea, just ask a Jew.
      Unfortunately for us all what it comes down to is that humans see a thing that is different and don’t understand. What is not understood is feared; what is feared becomes hated. Thus what we hate we destroy.

      1. I could not stop laughing while reading your reply Lauren. As the facts stand this is all there is XX and XY. Everything else is physical or mentally retardation, or people in serious need of professional counselling. Which is less than .001 of 1% of the world population, without even needing to look it up to do the research. Which sums everything both of you have said. Nice and tidy. Truth is if gender was fluid, then so is race, which then means both are only a state of mind. Which in that case I identify as a polka dotted apache helicopter, and you should respect that. Then I should also be able to say I identify as a woman to be able to apply for and get a small business administration loan for Women-owned businesses, from the government which is specifically an advantage only for women. From the SBA.gov. Funny part is that will never happen because people only want equal rights as long as it is in their favor. Fun world we live in. I probably wouldn’t have gone that far, but you brought in religion in your reply and lost credibility.

        1. You just said all non-binary individuals have mental retardation. Cute. I hope you share your opinion with everyone you can.

        2. Race is genetically non-existent though. Two identical-looking penguins might have more genetic variation between them than a black and white persons. Even “physical features” that distinguish specific “races” overlap across many “races.” In reality, people develop according to their ancestry’s geography, and these are merely small adjustments to survive in different climates (go closer to the equator for many generations and your family tree will develop a darker skin even if you somehow “maintained racial purity”) and conditions. Comparing race to gender is a waste of time from a scientific standpoint. Just tossing my two cents to the conversation.
          http://sitn.hms.harvard.edu/flash/2017/science-genetics-reshaping-race-debate-21st-century/

      2. Lauren, humans have diseases, we all know a man is XY and a woman is XX, if we find a XYZT that is not standard or normal, it is an abnormality.
        Bottom line is that people do not change who they are just by thinking or feeling sth.
        I can wake up tomorrow felling I am a Bear, dressing as a bear, hibernating as a bear but this DOES NOT make me a bear, I respect the feeling and freedom of a bear-man to be who he wants to be, but it is a disease and I am not obligated to call him Mr. BEAR and pretend I am blind

        1. This, exactly. What’s more, we should help the individuals adapt to society, not society adapt to them. If you had a glove factory, would you adjust things to produce 4 or 6 finger gloves because a tiny percentage of the population either is missing a finger or has an extra one? Of course not. Certainly, you might take them into consideration, and prompts to you for doing so, but changing the entire program of your factory to adjust to that minority would lead you to bankruptcy because the vast majority of people have 5 fingers. Furthermore, I certainly don’t want my kid chopping off a finger because he feels like he has 4 fingers and having a 5th disturbs him. Now imagine that the 5th finger is, you guessed it, genitals. Then again, I also don’t want my kid to hang himself because we just won’t accept that he’s a 4-finger-having kid. The situation isn’t simple, so we must be very careful about how we go about things when it comes to these issues.

          Before anyone jumps on this (always happens), full transparency, the gloves and fingers comparison is for simplicity’s sake. It obviously isn’t a perfect parallel, so don’t go thinking I’m trying to demean or brush off what trans people go through.

          1. Hey Carlos,
            I think your idea that we should be helping individuals to adapt to society is a great starting point, but I think your perspective on gender dysphoria is, willfully or otherwise, disrespectful. Though you did say “it’s not a perfect parallel”, it’s still your main argument, so let’s dive into your finger analogy.
            I can see how it’s compelling to think of in this manner – it’s a very cut and dry, material argument, especially if you substitute the fifth finger for genitals – but I think that’s the exact point you’re missing.
            Sex and gender are two different things. Sex – biological sex – is what this article is about. It’s the layout of XY or XX chromosomes and how they determine the biological structures of your body. Gender is a set of societal expectations and – most importantly – self understanding. For many, this does stem from your biological sex, to some degree, but the science is set that “gender identity” is both real and exists on a spectrum. Studies show that personal gender identity is typically settled upon between 2-3 years of age, a year or two after the child recognizes a difference between genders, and does not usually change.
            To think that your son “feels like” a 4 fingered person, or “feels like” a bear is to equate a physiological fact of your son’s brain to a “feeling”, like one feels sad, lost, or frustrated. This is false equivalence. Gender dysphoria is not a trend, and not just a feeling. People do not go “trans” because it’s trendy, cool, or because everyone is doing it – that’s nonsense fearmongering.
            That would be like saying sociopaths simply don’t feel like having empathy, so they have turned it off. Or that someone with depression is just “feeling sad”, and really ought to quit it. It’s an outright denial of the concrete, material circumstance of the person’s mental experience.
            Let’s go along with the ludicrous metaphor and make it a little more equivalent, and hopefully we can come to a common understanding.
            Let’s say for the sake of argument that 4-fingered and 5-fingered people really are treated entirely different by society, that no one accidentally loses fingers, that the idea that the number of fingers you have is a part of anyone’s identity whatsoever, that there is a legitimately recognized psychological occurrence of people experiencing 4-finger dysphoria. Let’s also say that in the US, 1.5 million people feel the same way as your son – having been born with 5 fingers, but feel not at home in their own bodies because of the extra finger they have.
            Sure, your glove factory will most likely continue to produce 5 fingered gloves because, it is a factory, and making money is the most important thing. Systems that aim to make money tend to target the largest possible crowd – though a smart company (or startup that identified this niche in the market) might start selling 4 finger gloves at a smaller scale to capitalize on that factory as well.
            But who cares what a factory would do? Our government, and our personal lives, are not run like a factory would be. Our government should still pass laws stating that it’s illegal to discriminate against someone with 4 fingers, or 4 finger dysphoria. Our government should still pass laws saying that 4-fingered people are allowed to be in the millitary, despite perhaps having slightly worse gun-handling. Our government should still recognize, protect, and serve those members of our community, the same as you should, because you’re a human being – presumably with empathy.
            I feel like I should finish this by letting you know – the suicide rates of the trans community are largely attributed to the way they are treated. Hate crimes, discrimination, and the opinions of people like Ricardo, Jake, and Will – cisgendered folks who think that “the way things have been” is enough evidence to discredit the entire lived experience of hundreds of thousands of people they have never met.
            In short, I hope that if your kid – hypothetical or otherwise – truly does wind up questioning the difference between their gender identity and assigned sex, you don’t treat him the way this opinion implies you should. Being trans is not a disease, getting transitional surgery is not the same as cutting off a finger, and this sort of disregard from someone who isn’t directly affected by the subject is chilling to hear.
            I don’t mean to “jump” on you. You seem like a nice enough guy. I just want you to really think twice about your preconceptions about this topic. It doesn’t cost you anything to do.

    2. So Will what if you were born with a vagina and have a XY chromosome?

      What if you were born with a penis and have a XX chromosome?

      Than what’s your gender?

        1. Not technical or scientific, I am a woman of 59, three children.
          I am xy, I have an introverted womb. Too much facial hair, normal puberty development etc. I do have family members of both sexes who identify as opposite, both male and female. So there is a lot more to it than than just xy…… I summise that other genes switch the sry gene on…. Mine are just broken

    3. Will, that you have a medical degree and are clearly against expanding your knowledge and learning continuation is terrifying.

  2. A couple different thoughts here, and I hope I can keep them coherent. All of the discussion of sex determination is interesting. I understand that certain species of plants and animals use different systems to determine biological sex. Also, that certain systems can be non-binary, etc. That being said, we are are not talking about bees, or melons, or fish or turtles. We are talking about human beings. XXY and XYY are mutations and defects of the genetic code. To use these disorders in an argument for non-binary gender is intellectually dishonest. The larger problem here really has nothing to do with gender. For the most part it is still OK to shame people for being fat, or poor, or not as smart, or just plain different. People will make fun and discriminate based on so many things. Growing up is hard, adolescence is hard, and there are a lot of challenges out there. Legislation that singles out trans people for humiliation are not good laws and are not good for society. That being said, I do not believe in non-binary fluid gender. I hate the use of the word “assigned” when talking about biological sex. Physically, you are either a boy or a girl, and anything else is an anomaly. But it’s probably good policy to have gender neutral bathrooms. It makes the line go more quickly. To Will’s point, my niece claimed that she is really a boy, and is transitioning FTM. She was always saying that she is lost, not fitting in, and that changing her gender would make everything better. She is an 18 year old girl, still developing as a person but now taking a whole bunch of hormones. And guess what? She is still lost, and doesn’t know what she is going to be doing with her life. Furthermore, the hormones have made her snarky and straight up obnoxious. Not fitting in can be very painful, but it would seem to me that when those around you don’t know who/what you are – that isolation would seem to be more painful. If you are feeling insecure about your identity, learn to accept yourself for who you are not transition into something that you are not. If the Queen of England had balls, then she would be the King……

  3. I greatly appreciate this blog post. It’s very informative.

    Is there a good journal article that summarizes the current science on sex determination in humans that deals with all the various issues like SRY migrating to the X chromosome that can give rise to XX males and similar situations where XY individuals can turn out to be biologically female?

  4. I agree with the gender as social construct thing. I don’t feel like a woman, or at least not the version of woman that society expects of me. In fact if anyone could tell me what it feels like to be a woman, please enlighten me. I am however a biological female and my life experiences are based on this fact. My hormones define my feelings, behaviour and actions, I cannot avoid it. As a biological female I have experienced inequalities based on having a vagina and a womb, not on what I identify as. My identity as a female is based on biology not on the whims of my mind. I don’t identify as a woman because woman is a gender construct and I wasn’t raised that way. That does not make me trans, a man in the wrong body, or any other such nonsense. I am still biologically female. And if I want to piss in a room that only allows other biological females in there why shouldn’t I be able to?

    1. Because being trans is not as u said…that’s all. This is much deeper than just not fitting into society. Even if we have one day equity into our society, trans would still be there. Because it has always existed, if u look into older societies we can find traces. But of course maybe there is people who r defining themselves as trans to escape the society, which is understandable. After all we r all different, let’s not forget that.
      Also, please note men will not being trans to rape anyone (ur case about bathroom seems so. Sorry). It costs way too much to do that. And they will not wait for that to rape anyone. Understand that.

      U feel u r a biological female, u r okay with this ? This is good for you. They don’t feel as they r in their defined gender, that’s all. And it is not based on society. They just know they r not fitting as u know u r fitting. This is THIS feeling u said. Please note this and don’t say this is ”nonsense”, try to hear them at least. To get a real idea of it. This is really dangerous to not help them.

  5. Okay, there are a few things I would like to clarify. One is that there are many gender abnormalities that happen, one of which is Klinefelters syndrome, which can still be identified genetically. What cannot be identified genetically is someone that is born as a girl with XX chromosomes and then somehow the girl is a boy. All forms of gender can be identified through an individuals genetic code and in the case of Klinefelters syndrome, it happens in .004 percent of the population. It is a medical anomaly as is any other genetic offset.

    Now, if an XY decides they are a girl and wants to have an operation to make themselves a girl, it does not change the fact that every cell in their body still identifies genetically as a male except for maybe some of their sperm cells. Furthermore, the discussion about equality is absurd. When males that decide they are female start raiding the female sports world, you will understand what I am talking about. But the inequalities work in both ways and neither sex is better than the other. Men are no better than women, and studies show that women are better at multi-tasking, tasks that involve social interaction, are more apt to be artistic, etc. Men really just carry the genetic strength that comes from the hormone difference of testosterone.

    There was a story I read about a 42 year old man trying to use a defense in court that he identified as a 14 year old girl. He was having sex with a 14 year old girl, so this whole thing was basically him declaring himself insane. He lost the case but the idea that an individual can identify as they please is insane. And as it is becoming more common, there are people that had spent their entire life identifying as a heterosexual female that all the sudden are a pansexual male because the people around them experimented with sex and determined that they have to be different. It was not a revelation, it was just someone feeling left out and wanting to be part of the what is going on.

    What makes my entire comment said is that someone will be offended by it. Genetics is able to tell the individuals sex and it has been that way for over a century. Stop trying to give excuses for people to identify in 400 different ways because it makes them feel better.

  6. A very interesting article!

    Everyone seems to be focused on the possible political side of this posting. I understand there is an undertone of that, particularly with some of the intro, but what I got out of it was a fascinating documentation of reproductive methods across the animal kingdom. It even touched on cantaloupes at one point!

    I’m sure there’s much we don’t understand on these processes, and possible some things we have wrong. As with almost everything. It is amazing how many different systems there are in place for different types of animals. The honeybees and tortoise variation on what affected outcome of sex was something I’d never heard of before and was a bit dumbfounding to me. I actually laughed out loud when it was mentioned that Platypus have 5 pairs of sex chromosomes. I figured, of course they would. Very strange animals.

    It looks like the second link you’ve provided is the source material. I’ll be giving it a look.
    In the chance that is not your source material: citations would be appreciated.

    Thank you for the informative and entertaining writing.

  7. Contributing medically! sex determination system in a genome and how chromosome(xy and xx) are formed in male/female provided sperm and ovum during interface could tell that a supernatural being really molded, designed and created these hormones in human body!

  8. This was a great article to read. It is information rich while still being very enjoyable, which is not the case with all scientific papers. Dear God, some of the papers and books are so dry and dense it’s almost impossible to get through them. This is such an excellent blend of deep scientific research and juicy entertaining writing. Thank you.

  9. People are forgetting how genes play in developing genitals, not just chromosomes. The one and off switch…

    This along brings a lot of possibilities in how hormones can change things

  10. McGill University. “DNA Not The Same In Every Cell Of Body: Major Genetic Differences Between Blood And Tissue Cells Revealed.” ScienceDaily. ScienceDaily, 16 July 2009. <www.sciencedaily.com/releas

  11. Fascinating article and the comments have been equally enlightening. I’ve long felt I understood why sexual reproduction evolved, but now I better understand it’s also really an umbrella term for a host of different variations that nature has explored.

    Sexual dimorphism is a reality that helped the human species to survive a dangerous world. Otherwise we wouldn’t be here or would be a different kind of organism. Most human beings now live in less brutal times, with a long life span and minimal existential threat. The full possibility of gender identity, once unexpressed in the struggle to survive, emerges. We understand more about all the variations on the human theme that are possible, even if such a variation occurs in less than one percent of the population. We can acknowledge that nature ‘s strategy is to make XX and XY people, but the creation process isn’t perfect, and then 9 long months of fetal development adds more variables. So the question for society is, now that life is about more than survival and we have become more aware of variations on the human theme, how do we define human rights for everyone. Along with this, how do we construct conditions for a good outcome of future civilization and avoid stagnation or self-destruction. How to allow all people to be the best, happiest, most self-fulfilled versions of themselves, while making sure society also functions well and is productive and advancing.

    Statistically, it does certainly appear nature is trying to make people into two sexes. But as human society advances, it also has to acknowledge the complexity and fuzziness that surrounds this rule. As trillions of tiny building blocks of life self-assemble, it doesn’t always go according to plan. The ramifications of acknowledging this “fuzziness” are many, and demand us to think and perhaps be ready to challenge our own assumptions of who to accept in our human tribe. In the wisdom of acknowledging the complexity of life, we can acknowledge gender dysphoria or gender fluidity as possible outcomes of human development, even if statistically somewhat rare outcomes. This acceptance shows scientific understanding and, moreover, compassion borne of a more inclusive understanding of our human family. However in this fuzzy, but generally more inclusive arena, the challenge is where to deploy new objective definitions of gender status so laws and protections can be written for the greatest good. In our wider accommodations of gender, I personally don’t believe we should descend into total relativism regarding gender identity, decoupling it completely from biology. I think this would be exploitable and potentially destabilizing to the ideal goal of a high-functioning society. At one extreme, society can no longer humanely ignore the full spectrum of gender outcomes, but at the other can’t function by granting rights to all possible self-proscribed identity realities. Thus, for example, allowing XY trans-females competing with XX females is a rightly debatable decision. It speaks directly to the hard realities confronted in that gray area. “It’s not fair to true women.” “Well wait a minute, what is the definition of a woman?” Perhaps as gender re-assignment technology improves in the future, we will be able to minimize the XY/XX phenotype difference to a degree where much of that particular debate is not an issue. Of course, once we are in that brave new world, new debates about what it means to be human will make the gender argument seem quaint.

    For now the effort continues to define a world with the most good for the most people, given the knowledge of the time. Hopefully, with a more widespread and nuanced understanding of the complexity inherent in human evolution and development, it will lead toward more inclusiveness of variation on the human theme.

  12. At 5, realizing the clothes, types of toys, underwear I would have to endure through this new journey I found myself in, was going to be lifelong, no changing it. I was going to be a girl, always, and that sucked. When my mom cut my hair (dad said it was always messy, so cut it) I was ecstatic! People in stores thought i was a boy and i didn’t correct them. Going into new schools, kids would tell me later, they thought i was a boy by “the way I walked”. I rode a minibike before a bicycle at 6. Easter was torture being forced to wear a scratchy dress. I was always jealous of the boys getting the better toys, having short hair, and having muscles without really trying. This isn’t about sexual attraction as all of this was going on before high school! With that said, I had little crushes on my pretty teachers, older sister’s friends, then Jodie Foster, Madonna, etc. Dad taught me the world is narrow-minded, so go ahead and wear baseball hats, no makeup, and boots all your life, that’s your prerogative, but don’t be surprised if you lose opportunity. Harsh? Maybe, but this one journey around the earth was the best, once I made the best of it.
    I would never want to oppress myself by taking extra hormones and shots, cutting off body parts, or adding them. Who knows how this will affect your body, bones, nerves, endocrine system, immune system, neurons, brain later?
    Many people aren’t their ideal self, have disabilities, or dealt a hand of bad health. There’s blindness, MS, Cushings Syndrome, Diabetes, dwarfism or hundredsof other permanent anomolies. They had to make the best of it, and had to work around the society of the time, why shouldn’t I? Hollywood stars with lots of money,
    no contentment may want to spend their way into happiness thinking they can jusr become the sex they felt like. But to me, this is a terrible role model for our young society. They have the money to fix it if they get it wrong, or it doesn’t make them happy, where many young people, especially lower income, cannot. Do we really know how many transgender people really become happy after radically altering their body, and hormones? We need to grow as a society by ACCEPTING one another, in the part of the spectrum we were born at. Men in dresses and makeup, with a penis, could be normal. Women in baseball caps, or 3-piece suit is normal and ok.
    Then we can truly teach our youth about acceptance and loving themselves.
    I KNOW my brain has always been different than my body, and I don’t know why, but maybe this is evidence of 2 more genders, a brain/body opposite and if society would understand this, people wouldn’t feel they had to change their outside to match as we would all be “normal”.
    It’s the body God gave me and, thankfully, my dad told me how the world was going to be in my one and only little girl journey on this Earth.

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