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My understanding was that XX/XY was how sex is determined in mammals, and so humans as well.

A science teacher posted this regarding transphobia:

"I just commented this on a transphobic post that was all like, "In a sexual species, females have two X chromosomes and males have an X and a Y, I'm not a bigot it's just science." I'm a science teacher so I responded with this. First of all, in a sexual species, you can have females be XX and males be X (insects), you can have females be ZW and males be ZZ (birds), you can have females be females because they developed in a warm environment and males be males because they developed in a cool environment (reptiles), you can have females be females because they lost a penis sword fighting contest (some flatworms), you can have males be males because they were born female, but changed sexes because the only male in their group died (parrotfish and clownfish), you can have males look and act like females because they are trying to get close enough to actual females to mate with them (cuttlefish, bluegills, others), or you can be one of thousands of sexes (slime mold, some mushrooms.) Oh, did you mean humans? Oh ok then. You can be male because you were born female, but you have 5-alphareductase deficiency and so you grew a penis at age 12. You can be female because you have an X and a Y chromosome but you are insensitive to androgens, and so you have a female body. You can be female because you have an X and a Y chromosome but your Y is missing the SRY gene, and so you have a female body. You can be male because you have two X chromosomes, but one of your X's HAS an SRY gene, and so you have a male body. You can be male because you have two X chromosomes- but also a Y. You can be female because you have only one X chromosome at all. And you can be male because you have two X chromosomes, but your heart and brain are male. And vice - effing - versa. Don't use science to justify your bigotry. The world is way too weird for that shit."

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closed as unclear what you're asking by theforestecologist, James, AliceD, kmm, David Mar 30 '17 at 13:06

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Please format and clarify your question. $\endgroup$ – canadianer Mar 5 '17 at 20:25
  • $\begingroup$ Welcome to Biology.SE. We are a bit picky on SE sites and like that OP make a good effort when asking a question. You might for example use > to highlight the quote. You might want to reduce the quote (eventually using [..]) as it is quite long. You should restate your question in the body of the post. As the quote goes in a number of different directions, you should very clearly link your question to the quote. $\endgroup$ – Remi.b Mar 5 '17 at 21:21
  • $\begingroup$ Upvoted just for the last couple of sentences :-) $\endgroup$ – jamesqf Mar 6 '17 at 2:05
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    $\begingroup$ This question, whilst delivering a very positive message, is long and I'm not sure exactly what the question is? Could OP clarify and explain specifically what they want to know after the quote. $\endgroup$ – James Mar 6 '17 at 3:30
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    $\begingroup$ This question is combining different arguments: human gender based on genetics vs. based on appearances vs based on culture. This is too broad, and can likely only lead to open-ended discussion. In fact the question is unclear and this post seems to simply be seeking discussion. I'm voting to close as too broad and as being unclear what you're actually asking. $\endgroup$ – theforestecologist Mar 14 '17 at 16:56
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If you're asking if humans are a sexually dimorphic species, the answer is "yes". If you're asking based on the viral response of the science teacher, then the answer is still, "yes". What is described above are chromosomal abnormalities, or other disorders of development, but they are still describing sexual dimorphism in the phenotype. However, the answer itself is incomplete, as it's written out of political motivation. Whilst true that a person can present, phenotypically, the opposite of their genetic sex, this isn't akin to them "being male/female" counter to their allosomes. Of course, this in itself can prove to be a contentious issue - how is sex defined? Under normal circumstances, phenotype is the obvious way of knowing. If someone has the external genitalia of a male, they can be assumed to be male, and vice-versa for females. When it comes to chromosomal abnormalities, there is an "error" in devlopment, where the genotype is not properly expressed in the phenotype, and so while presenting as one sex, the "true" sex as determined by the genotype is the opposite. It should also be noted that sexual characteristics can be ambiguous certain cases, creating abnormal sexual presentation, such as in hermaphroditism/pseudohermaphroditism, or "intersex" (the quote mentions 5-alpha-reductase deficiency, which is associated with male pseudohermaphroditism).

The problem with the quote above (at least in the way it is written) is it appears to suggest that chromosomes have little to do with phenotype, as in sex determination, and that this carries over to gender dysmorphic patients. These are both mostly false. Gender dysmorphic patients generally do not have any allosome abnormalities, and the reason that the given examples are deemed "errors" (eg XX male syndrome; 47,XXY) is because that is what they are, as well as being rare. The typical effect of these problems is that the patient is infertile, (i.e. they are not sexually functioning, and have reduced biological fitness).

In short, the teacher is correct in the examples used, but overstating them with regard to the general population, and with regard to transphobia and gender dysphoria. Humans are indeed sexually dimorphic but as with any complex biological system, errors expressing genotype can occur.

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