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In a near future scientists have managed to totally delay the sexual differentiation in man. The children are born without any primary gender, just as a fetus in the womb before the differentiation into male or female takes place. Also there is no genetic difference in chromosomes or anything else that differs between the sexes. At a certain age, about 15-18 years, the youngster has to decide what sex it wants to be, and a medical intervention of some kind takes place. The youngster then quite rapidly develops into a man or a woman.

Of course this costs a lot of money and is government financed in most rich countries, so no one is left untreated there. The case is otherwise in the third world where a lot of children still have to be born with a discernible sex. Luckily there are UN help workers who try to correct this.

My Q is about the most plausible way to achieve this delay and how to medically trigger the process when the kids get older. Gene manipulation? Hormone treatment? Mass radiation??

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    $\begingroup$ There will be problems with ovaries. These needs to develop in fetus, I'm afraid. So tomboy girls, half of them deciding to be guys, seems more probable. $\endgroup$ – Mołot Oct 21 '16 at 12:09
  • $\begingroup$ @Mołot That wouldn't have the same problem as a prostate? $\endgroup$ – corsiKa Oct 21 '16 at 14:53
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    $\begingroup$ No. Prostate is not creating sperm cells. And these are created during guy's lifetime. Girl is born with all eggs she will ever have. $\endgroup$ – Mołot Oct 21 '16 at 16:00
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    $\begingroup$ What you contemplate is a completely new species. The term inhuman springs to mind. $\endgroup$ – Karl Oct 21 '16 at 18:51
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    $\begingroup$ There are a lot more things that would be affected by something like this than I care to write about. The sex cells aren't even close to an issue in reality. The problem comes from all the secondary things that go along with sexual dimorphism which effects many many things. If the goal is being able to choose sex, then it would be best to just use the gene therapy when they reach adulthood. If you do it the otherway you're just looking at lots of moral, ethical, and practical issues from both a biological and cultural stance. $\endgroup$ – Durakken Oct 21 '16 at 21:40
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The delay is not a problem. Caveat: Providing you can decouple it from the development of the urinary tract - being born without an ability to pee would be fatal.

All humans start out with a proto-gonad - the gonadal ridge. This can become either a testis or ovary. All humans start out with both the Mullerian (female) and Wolffian (male) duct system. Usually one of these gets reabsorbed and the other develops fully. Wikipedia page with a good summary.

Some hand-waved genetic engineering can slow the above processes down, or bring them to a complete halt. So just as real world human men don't grow beards until they are a certain age, your humans would not begin gonadal differentiation or duct formation until they are 15.

The chromosomes/genes are a problem. If you leave them as they are now, then the kids don't get to choose - they just hit 15 and start to become a man or a woman. It'll still change society radically, because unless you karyotype your kid or test for certain genes, you won't be able to do gender-stereotype child rearing. ("Oh dear, I called my child Mary, dressed it in pink and refused to let it play with toy soldiers, but turns out it's a man!")

I don't know enough genetics to determine if any of the following is plausible. Mammal chromosomes do screwed up things when they haven't been 'tagged' as coming from a mum or a dad (epigenetics).

Soooo... rebuild your genome. No X and Y. Create a new Z chromosomes (so everyone is ZZ), which contain all the male and all the female differentiation genes. Ditto the autosomes (there are sex determining genes on chromosomes other than the X & Y - read Sex Itself by Sarah Richardson to see just how complex sex determination in humans really is).

You'll also have to sprinkle your new Z chromosomes with genes which switch on and/or off all those 'male' and 'female' genes.

Hand wavy genetic engineering again. Have the trigger to activate the first of the 'switch on' genes (and the following cascade effect) as some chemical/s which are not commonly available in the human diet, or which are normally at much lower doses. Like if you want to be a woman you have to eat 1kg of royal jelly and spend a week on a so-high-its-almost-dangerous vitamin A diet.

Downside: if you need medical intervention to produce fertile adults, you'd better hope there is never a crisis which reduces your tech level below what is needed. Also if there is a royal jelly shortage, you can't produce women.

Adolescence is going to be hell. First you have to grow all the plumbing, then the usual raging hormones and sexual maturation stuff happens. Maybe you could do the adolescent brain re-wiring (intellectual leaps, etc) separately from the sexual maturation? After all, they'd be better able to understand the ramifications of their choice if they were doing it with an adult's brain, not a child's brain.

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Design a new genetic creature, that grows in the way desired, which happens to seem like a human, and may have several genetic similarities (but probably can't breed with humans). You would then attach the desired genitalia, factory made, when they make their choice. Or, just design them to be able to grow genitalia to their desires. That's the simple way of doing it.

If you wish to see an example of a true hermaphrodite, look up XXY chromosome syndrome: https://en.wikipedia.org/wiki/True_hermaphroditism https://en.wikipedia.org/wiki/Klinefelter_syndrome

Only one set of the sexual organs has even been functional, but therapy has been able to steer them biologically more towards one than the other. Your chances of having kids will be very slim, and there are some other health issues. However, this process has to start from child birth to have a decent chance of success. Gene therapy may be able to deal with some of these issues.

If you want to go very high tech, you could use this condition, then neuter the subject (and hopefully have more effective hormone therapy). Then, you would manufacture sexual organs with the host's DNA from proteins, then attach them to the host.

There isn't much other plausible way of doing it. You would still likely suffer a lot of health defects. The alternative is to rewrite a lot of DNA code in order to remove all of them. The issue is, XXY syndrome isn't exactly a specific gene, it is the issue of too many conflicting genes telling the body what to do. It's a defect that renders the subject both male and female, and effectively neither male nor female. It's a devastating defect that is hard to treat and harder to live with.

Look into amphibian DNA. A number of amphibians can change gender as adults. It might be a slight issue, that their kid will have to lay eggs in the water to reproduce... but that's the price you pay for such a feature.

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    $\begingroup$ Amphibian DNA appeals to me. Of course there is a price to pay, and that might be what the story is about. $\endgroup$ – Beta Oct 21 '16 at 11:04
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    $\begingroup$ Conflict is after all, the heart of engagement. Wish you the best! $\endgroup$ – J. Doe Oct 21 '16 at 11:06
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    $\begingroup$ And don’t forget clownfish! I’m disappointed that the adult clownfish in Finding Dory hadn’t become female since the first movie. $\endgroup$ – JDługosz Oct 21 '16 at 19:12
  • $\begingroup$ @JDługosz I like that idea, too. However clownfish start their lives with one sex. The children in the story have NO sex at all. $\endgroup$ – Beta Oct 21 '16 at 20:30
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It's really hard to suppress chromosomal development, because if you don't have either XX or XY, you're most likely sterile. However, you can make it so all humans are always born with the same chromosome. You can choose XY, by injecting fathers with a spermicide that specifically targets X sperms; and vice versa you can force all humans to be born XX similarly by suppressing Y sperms. A bloodier, but much cheaper way to do this is to simply abort or kill all fetuses/babies with the "defective" chromosomes. There are advantages and disadvantages of whether you choose XX or XY. If the chosen chromosome is to be XY, it will be easier to revert to mixed chromosome culture by stopping treatments, but if you choose XX, there will be no turning back once the last XY died. So in the long run, maintaining XX-only civilisation is easier.

You can use hormonal manipulation to suppresses the hormones that causes the development of primary and secondary sexual characteristic. According to Wikipedia, development of primary sexual characteristic is controlled by hormones:

In the first weeks of life, a fetus has no anatomic or hormonal sex, and only a karyotype distinguishes male from female. Specific genes induce gonadal differences, which produce hormonal differences, which cause anatomic differences, leading to psychological and behavioral differences, some of which are innate and some induced by the social environment.

If your chosen chromosome is XY, you would want to decide whether you want to mess with the TDF/SRY gene. If you suppress TDF/SRY before gonodal differentiation, you'll trigger development of female sex organs, so all children will be genetically XY/male but physiologically female and have female reproductive organs, so those wanting to be male will need operations. If you don't suppress TDF/SRY, all children will be essentially male and those that chooses to be female will need operation.

Male-to-female surgery seems to be more expensive than female-to-male surgery, according to CostHelper at The Philadelphia Center for Transgender Surgery:

basic male-to-female surgery ... would cost a little over \$30,000 while for basic female-to-male surgery ... would cost between \$12,000 and \$25,000 depending on the type of genital surgery chosen.

For the cost-conscious governments, it seems that it'll be cheaper and preferable to have physiologically female children where half of the population will receive female-to-male gender reassignment surgery.

If you want to require medical intervention for both genders rather than just one, then you'll have to find a way you suppress the development of both male and female reproductive organs. In XY only population with TDF suppressed and in XX only population, you need to suppress the hormones that triggers vaginal development. This is to keep both pre-penis and pre-vagina cells undeveloped. In adulthood, you'll either artificially trigger pre-penis cells to start developing or undo the suppression of the hormones that triggers vaginal development. It's hard to know whether it would've been possible to trigger the development of primary sex organs at such a late age. Once the primary sex organ develops, the secondary sex organs should follow suit as they depend on the hormones produced by the primary sex organs.

Another complication for XY only population is that, since the mothers also have XY gene, then half of her egg will contain Y egg. That may cause two things, either reduced fertility because Y egg is either not viable or can only mate with X sperm. I don't know what would've happened with those born to the YY chromosome, they'll probably be non-viable. Also, if you want to maintain XY only population, you'll still have to suppress/abort all XX conception. This leads to a population with at best half the fertility of mixed chromosomes population. No such issues exists with XX only populations.

Note: I'm neither an expert in human reproductive system nor biology, so even though I try to base my answer on the sciences as I understand it, at least half of the things I said here is probably scientifically wrong.

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  • $\begingroup$ Rather than killing the Y chromosome, you could mutate it into a hybrid "Z" chromosome, with XZ leading to hermaphroditic / intersex physiology, while XX leads as now to (mostly) female, and ZZ to (mostly) male. XZ would become the most common (random pairings would result in a ratio 18XZ:5XX:5ZZ, with XX+XX and ZZ+ZZ pairings physiologically impossible), and might be further selected in rich countries by mass adoption of IVF or other medical pre-selection / screening. $\endgroup$ – IMSoP Oct 23 '16 at 15:21
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Delaying the Onset of Sexual Differentiation

To get viable ideas, we only have to look to nature.

  1. Some animals, like frogs, have to have the developing zygote sitting at a very particular angle.

  2. Some animals, like some fish, are able to change genders during their lifetime. It depends on the number of males to females in the population that drives it.

  3. Some sex selection, maybe alligators or turtles, depends on temperature (e.g. the genes that determine sex selection are only active at a particular temperature, so cold day you get males, warm day you get females (or vice versa).
  4. Maybe the genes for sex selection are multivariate and assort with other genes, so sexes are highly dimorphic.
  5. Maybe the genes that determine sex are located within a transposon, a genetic element that can rewrite itself in other places of the genome, and people randomly change sexes, possibly several times, through their life.
  6. Maybe in the line of menses, there is, say, a calorie counter gene/molecule/protein that when the appropriate amount of nutrients have been consumed (or correct amount of UV light, or whatever) then it differentially activates a vast panaply of genes that guide sex selection.
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If male humans are lacking enzyme 5-reductase they're guevedoces, who get male genitalia at the age of 7-12; otherwise look like females.

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