How would "Amazon" (a female only subspecies) genetics work?

Question

The traditional Amazon tribe in mythology is a society of only women that reproduces by capturing outside men. Suppose that the Amazons were an actual sub-population/subspecies of humans that, as the mythology goes, was all female and could only reproduce by capturing outside ("normal" human) males. What sort of genetics could produce such a phenomenon?

My naive approach would be to suppose that Amazons have a modified X chromosome: X! that causes their offspring to only be female.

So an Amazon mother (X!X!) and a human father (XY) would produce:

• X!X "mixed" Amazon
• X!Y non-viable

But then the problem is that a "mixed" Amazon would then go on to produce normal human daughters and sons since if you pair X!X with XY you get:

• X!X "mixed" Amazon
• X!Y non-viable
• XX normal female human
• XY normal male human

So this approach doesn't seem to work unless there's some way for the X! chromosome to overwrite the X chromosome from the father (turning X!X into X!X!) during conception. But is that possible?

Answer 1

Driver chromosome.

Your Amazons carry a driver chromosome. Driver chromosomes are examples of "selfish" genetic elements which suppress their competition during meiosis.

X chromosome drive in a widespread Palearctic woodland fly, Drosophila testacea

X chromosome drive (X drive) provides a clear example of selfish genetic behaviour. This phenomenon was first described in Drosophila (where it is called the sex ratio trait) (Gershenson, 1928), but has since been found in a wide range of taxa, including rodents, plants and numerous flies (Diptera) (Jaenike, 2001). Broadly, X drive is characterized by the unequal transmission of the X chromosome, which can be achieved in a number of ways. In Diptera, X drive is achieved through the action of an X‐linked gene product that destroys or incapacitates nearly all of an individual's Y‐bearing sperm during gametogenesis. As a result, males carrying a driving X chromosome (XSR) produce predominantly X‐bearing gametes and therefore sire almost exclusively daughters (Jaenike, 2001).

Your X! chromosome is a driver. In fly males, the competition for the driver X during meiosis is the Y. The driver X in flies ensures all sperm are X. In your human females, competition for the driver X! is the normal X. The driver X! in humans ensures that all eggs are X!.

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Androgen insensitivity.

And here is the mechanism by which there are no males. Male pregnancies do not miscarry. They just are phenotypically female. The X! carries the gene for androgen insensitivity syndrome.

Androgen insensitivity syndrome

Persons with this X linked syndrome have an XY genotype but an XX phenotype. At its extreme, these persons physically are normal females but they do not have a uterus or ovaries, do not menstruate and of course do not get pregnant.

Answer 2

Gynogenesis

There's a thing in fishes where all-female parthenogenetic species of mollies cannot produce offspring on their own, but can only reproduce after being inseminated by a male. The sperm does not actually penetrate the egg, merely activate it so it can begin development and perform parthenogenesis on its own. The male does not benefit from this act, does not pass on its genes, and overall it's a waste of time and effort for the male. Studies have demonstrated that male mollies prefer to mate with females of their own species and avoid the parthenogenetic ones, so the parthenogenetic mollies have to trick the males into mating with them.

There are some variants of this phenomenon that do take in genetic input from the male, but end up "spitting out" the male sex chromosome, such that the offspring are always female. I don't know as much about these, though.

There is some similar behavior among other parthenogenetic species. Some parthenogenetic species like whiptails still require sexual stimulation in order to ovulate, so females end up mounting females to produce eggs. The mounting female still provides no genetic input into the future offspring, however.

Answer 3

Normal women and Amazon women are essentially the same, since they can both reproduce with human males.

The solution is therefore cultural.

Whenever a boy is born, because one of his ancestors was a normal woman, that boy will be abandoned into a bordering tribe, and he will be accepted as compensation for the males taken by the Amazons.

Don't forget that in China the single kid policy together with the preference for a male son has skewed the balance between the sexes toward the male, so it is totally plausible that a cultural approach controls the reproduction outcome.

Answer 4

It's Mitochondria, not Genetics

Mitochondrial DNA is inherited from the mother, so handed down from the Amazon mother to her children. Happily, mitochondria are also the energy source for cells in human biology, so some mutation that allows for super-powered mitochondria would then inherit from mother to child.

It's up to you what to do when the boys that are born. You could easily make some genetic reason for them to be non-viable, it just doesn't work with the Y-chromosome. That would also mean either a lower conception rate (50% less viable implantantions), or a much higher miscarriage rate. That may or may not be relevant in your text.

Answer 5

Trigger-happy genetic repair mechanisms.

Somewhat fantasy-based answer ahead, but with a dash of realism

The body has several mechanisms in which it can repair DNA. Your amazons have an exclusive, specially trigger-happy one, that sees most of the non-amazon DNA as "borked" and fixes it by making it more similar to the amazonian DNA - a type of Homing endonucleases

This mechanism enables amazons to be healthier and less vulnerable to diseases that come from genetic damage. It would reduce the chance of them getting cancer, reduce their rate of aging, and enable them to survive in ambients with higher radiation.

However, the same mechanism also ends up "correcting" the Y chromosome from their male partner, effectively rebuilding it into a X chromosome, and thus creating an all-female population.

Answer 6

If you are more willing to make your race more genetically distinct, I have another answer. Your human (sub-)species developed parthenogenesis. Parthenogenesis is:

Parthenogenesis is a natural form of asexual reproduction in which growth and development of embryos occur without fertilization by sperm. In animals, parthenogenesis means development of an embryo from an unfertilized egg cell. In plants parthenogenesis is a component process of apomixis.

If parthenogenesis happened in human, the resulting gender has to be female since female doesn't have Y-chromosome to make a male baby.

This answer works well with L.Dutch's answer. Because you can't have a male baby from parthenogenesis, if someone bears a male baby, it will be a clear sign that the female has been raped or otherwise have sex with the outsider, so the baby is cast outside as not a part of the tribe. (Especially effective if the surrounding tribe is patriarchal)

As for how humans might acquire parthenogenesis, it might originally a random chance. But then since female parthenogenetic gets outcast (as they are unfamiliar with parthenogenesis), the outcasts formed their own tribe.

Answer 7

If what you want is an actual separate species or subspecies you have to have reproductive isolation which you will never get if they are kidnapping men to breed with, simple as that.

There's only one way to achieve an actual subspecies (something beginning on the path to actual speciation but not quite there yet) that's all female, parthenogenesis.

Anything where they breed with normal humans to get pregnant but only birth females (the effect of some form of dominant gene resulting in only female conceptions or early miscarriage or still birth of any males) isn't technically speaking a subspecies, it's the same species, same goes for if they simply get rid of male births by infanticide which would be cultural not genetics anyway.

The constant influx of DNA from other human populations in your example would ensure any form of speciation would never happen, simple as that.

So you're probably looking at some sort of random mutation as in whiptail lizards to start off with followed by cultural isolation ('men bad, we don't want men') to allow speciation (or sub speciation if you prefer) to begin to develop.

If you still really want them to kidnap men then try this on for size, Whiptail lizards often go through the motions of mating with each other even though no genetic material is exchanged (both being female), it's just an instinctive pantomime that helps get the hormones that trigger pregnancy going for them (those that do this tend to be more fertile), so perhaps have your Amazons fully speciated such that they can no longer breed with other humans, the sperm won't penetrate the egg, but the act of sex triggers hormonal changes making their parthenogenic pregnancy more likely & sex with men works better than doing it for yourself or with other women, they may not even realise the men are making zero contribution to the offspring, they could be fully speciated in this way & still look identical to normal humans.

So the question is do you really want a subspecies or just a cultural thing with a bit of a twist, which has already been done to death in fiction anyway.

Alternatively just have them lie about being a subspecies & not having any male births & toss any boy babies off the cliff out back of the house when no one's looking, much simpler.