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When looking exclusively at the muscular and skeletal systems of male and female humans, on average, we'd find what scientists call "sexual dimorphism". Even their voices are sexually dimorphic--males average on the tenor-bass range, whereas females average on the alto-soprano range.

In this science fiction scenario, a disgruntled and hostile ecoterrorist, guilty of illegally terraforming a series of alternate Earths, had somehow cleared up the whole of North America of its prisons and injected all of the man, woman and child inmates of one same mutation--the "banana slug lover", a mutation that results in everyone having the reproductive organs of both men and women. In short, turning all of North America's prisoners into simultaneous hermaphrodites. Once every single one had been injected and engineered, they get systematically released to fend for themselves in any one of the terraformed alternate Earths of the "scientist"'s choosing.

Once they settle, that is where the question comes into play. Will simultaneously hermaphroditic "men" and "women" still have the same kind of sexual dimorphism typical of other humans?

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    $\begingroup$ You have not specified what happens to the genotype of your victims, only their phenotype. Any change to the phenotype on this scale could erase sexual dimorphism, increase it, or do many other things. $\endgroup$ – John Dallman Mar 29 '20 at 14:38
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    $\begingroup$ a) Although the reproductive organs get all the press, glands and hormones are much more important to the rest of the body. b) Even if there were changes, it takes time for them to be realized throughout the body. c) Frankly, no matter what answer your story goes with, it will not be the most baffling thing about this premise by a long shot. $\endgroup$ – Cadence Mar 29 '20 at 15:15
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    $\begingroup$ The sexual dimorphism in humans is developmental, and it happens during puberty. If the transformed people have already gone through puberty, they will conserve the sexually dimorphic characteristics they have already acquired. If not, they won't acquire them. For example, the voices of male and female children are similar. The voices of men and women are different because during puberty the larynx of men moves lower; we say that a pubescent boy's voice "breaks"; it is triggered by the testosterone released when the boy enters puberty. See castrato. $\endgroup$ – AlexP Mar 29 '20 at 15:41
  • $\begingroup$ @Cadence ANY answer is better than NO answer at all. $\endgroup$ – JohnWDailey Mar 29 '20 at 17:14
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    $\begingroup$ I don't think this qualified for closing? to my mind it's a silly question & the answers are obvious to anyone with just a little research but I don't think it has any issues with 'detail or clarity'. $\endgroup$ – Pelinore Mar 29 '20 at 18:27
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I think to achieve its goal, the mad scientist would have created a new chromosome, H, to replace the X and Y chromosome. The H chromosome would provide most of the same genes as the X chromosome, allowing the bearer to live with an H instead of one or more X chromosomes.

One difficulty of producing functional simultaneous human hermaphrodites is that the female hormones (progesterone and estrogen) and the male hormones (testosterone) tend to interfere with one another. Even if an individual had both sets of genitalia, only one set could be functional. This new H chromosome would produce a new hormone, which I'll dub "androsterone". Androsterone would promote both male and female sexual traits, and would allow both types of genetalia to be funcitonal.

Depending on the details, the H could be incompatible with the X or Y chromosomes (inability to breed with standard humans) or it could dominate/partially dominate the X and Y chromosomes. It could be that HX and HY individuals would be sexually dimorphic (but still hermaphrodites), while HH would tend to be more androgynous.

Injecting this genetic change into existing adults and children will not cause them to suddenly sprout new genetalia; those grow in the womb. For the purpose of the story, the genetic change would probably cause the bodies to start producing androsterone. As the normal human hormones flushed from their systems to be replaced by androsterone, secondary sexual characteristics (breasts, beards, etc.) would start to emerge.

The next generation of people would all be hermaphrodites. Since they would all be HH, they would not be sexually dimorphic. Though, there could be variations on the H chromosome that lead to some level of dimorphism. Maybe there are two versions of the relevant genes, H and h, where HH tends to present as more female and hh as more male, though both combinations would still yield hermaphrodites. There are various modes of dominance, so Hh could be more androgynous, more one gender or the other, or in each individual, one could dominate (I believe that in women with two X chromosomes, one tends to completely dominate the other).

So, to explicitly answer the question: "Would there still be dimorphism?": The answer would be: "It could go either way, depending on the exact nature of the change."

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  • $\begingroup$ Answer accepted. $\endgroup$ – JohnWDailey Jul 20 '20 at 23:24
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If the genetic mutation is done through the insertion of a gene, you may cause everyone to change their phenotype towards androgyny. A simple microscope checkup of some mouth tissue will reveal the original biological sex of each person (female cells have a little dot in them, which is a "deactivated" X).

For adults, and those in their later teens, transformation may or may not change all sexual physical traits in such a way as to make their original sex detectable. Just look at transgender people - when the transition is done during early teen years and with proper hormonal treatment, they grow up to look more like the biological sex they identify with than if they start later in life.

So in short for the older people you might look at them and at a glance say "this one was originally a cis-man/cis-woman". For the ones in their teens you may need a throughout examination. For the ones who receive the mutation as children, years later you might need a microscope to determine their original biological sex.

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    $\begingroup$ Without surgical intervention there will be no change in an already developed adult organism, assuming this is some sort of virus born gene therapy there will be no new organs developed in those infected, they will retain their original phenotype, they effects will only appear in their offspring, in children infected pre puberty they won't develop a second set of sexual organs either but further development in puberty may be affected ~ offspring will be neither male nor female & likely share the same phenotype with little more dimorphism than expected within a single sex. $\endgroup$ – Pelinore Mar 29 '20 at 18:09
  • $\begingroup$ About the only phenotype change you might expect in adults is (depending on original sex) perhaps more or less facial & body hair & a change in body fat distribution, no new organs can be expected. $\endgroup$ – Pelinore Mar 29 '20 at 18:12
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    $\begingroup$ It's the same for prepubescent children (no new organs) accept that hair & body fat changes will likely be more pronounced, girls & boys will likely both develop breasts of around the same average size, reproductive organs won't change, though with the hormonal changes expected boys penises may be smaller on average. $\endgroup$ – Pelinore Mar 29 '20 at 18:16
  • $\begingroup$ @Pelinore I know but I'm taking into consideration the science-fiction tag. $\endgroup$ – The Square-Cube Law Mar 29 '20 at 18:47
  • $\begingroup$ Ahh! OK, I'll check the tags & reconsider in their light. $\endgroup$ – Pelinore Mar 29 '20 at 18:48
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If you only change the genitals then the dimorphism in adults will continue. Muscle development, breasts, and voice range are mostly developed in adolescence, so while the hormones would eventually correct that to some degree, the difference would remain and be noticeable. For children though (the next generation) since the hormones would be between male and female each would have roughly the same traits. so the first generation would have dimorphism, but the remaining generations would not.

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You have to follow the biological sciences here. They can possess both sets of genitals but only one set can be reproductive-functional. The reason is the hormones which are regulated by the hypothalamus (brain). It would produce one or the other but without serious reworking of human physiology you cannot explain reproduction of both hormones. For instance, progestin and estrogen are female - in sufficient quantities they would make your character appear female. Swap that around to testosterone and your character is growing a beard. They can't both be present as they sort of cancel one another out. That said, the brain can produce all three and men have a certain amount of female hormones and women a certain amount of male hormones - but one side has to dominate in order for reproductive processes to occur. Now you could of course create a hybrid alien creature which has some sort of dual-brain characteristic or get crazy where the character can shift hormone production at will, thereby shifting from male to female or something like that, but a normal human even with both sets of equipment, can't function as both at the same time. Not in a real world sense. But with fiction of course, you can just ignore that.

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I was gonna leave this as a comment... but let's face it, it's an Answer.

Given that our mad scientist named his mutation "banana slug lover" and, crucially, he is mad: you probably already know there will still be "Men" and "Women"... "once they settle", aka, you have expression of this new mutation in their offspring.

(and ya know, all of them being from prison probably doesn't help too much)

.... for those that don't know here's a dangerous linky

Although to more qualify as a dimorphism, testes might be included because that would change hormone balances. Then again maybe that's not strict enough to meet the definition of dimorphism. Up to you really. Anything more strictly defined as a dimorphism would have to be encoded into the mutation a la clownfish or one of those other ones with a more flip-flop sex mutation.

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