If you wanted to colonize other planets with meat suite humans, then females are the superior choice as they have lower nutritional needs. Here I imagine a fictional Amazon gene that could lead to a skewed sex ratio favoring females.


  • Human Females need less food than males
  • Human Females silence a random X chromosome because otherwise every protein from the X chromosome would be twice as abundant for XX females compared to as for XY males
  • If Humans could be engineered to have a female biased sex ratio, then they would have an advantage when traveling to new worlds because:
    • then they will need less food.
    • their space ships would require slightly less fuel as females tend to be lighter.
    • the could increase their population size faster as there are more gestating members of the species.

So this is a cool premise for an egalitarian feminist spacefaring society... or a harem anime :)


Imagine a novel X chromosome mutation that allows the X chromosome to silence the Y chromosome such that these XY mutants have a female phenotype and reproduction strategy, to make this less confusing we denote the mutated X chromosome as A, the Amazon chromosome

In this new society there are the following species with a male phenotype: XY, and the following with female phenotype XX, AX and AY (AA would also be female, but cannot exist as the males have no A's)

Try to stop up and guess the sex equilibrium ratio... Is it 1/4 of each one because there are now 4 sexes, is it 1/3 XY and 2/3 female genotypes?, or something completely different?

Amazon mating: The Next Generation

Let's try to "mate" these new sex genotypes and see what would be produced

XY and XX (the standard): XX has 100% chance of giving an X and XY has 50/50 for each, so the offspring will be 50% XY and 50% XX

XY + XX -> 50% XX and 50% XY

Let's try to mate XY with AX:

XY + AX ->25% XY, 25% AX, 25% XY and 25% AY

Finally XY with AY, if we assume that egg cells carrying a Y are nonviable, then AY amazons will always donate their A giving:

XY + AY -> 50% AX + 50X AY

We can put this in a table where the columns are the mating pairs and the rows are offspring probabilities. (For mathematical reasons explained later we add a Male on Male columns with has 0% chance of getting offspring)

XY 0 0.5 0.25 0
XX 0 0.5 0.25 0
AX 0 0 0.25 0.5
AY 0 0 0.25 0.5

Looking at this table, do you now have a new guess for the equilibrium sex ratio?

Lets say the space ship starts with 2 XY males and 8 AX Amazons, let's calculate the expected number for each of the 4 genotypes after 1 generation (assuming each female mates twice). We can express the 10 astronauts as a vector where items of the vector corresponds to the index in the table above, so like this

If we also consider the first row of the table as a vector, then we can multiply the two vectors as follows:

This is called the vector dot product, if we "keep sliding" the vector down the table, then it is called a matrix dot product, Thus to calculate the next generation genotype vector we simply need to slide trough the table (and multiply by the number of times the females mates, which is 2)

So after 1 generation there are now 16 people and a sex ratio of 1:3, and a 1:1:1:1 genotype distribution. One generation later we have:

Interesting The same distribution, thus if the Amazons decide to get only kids per female they will have a stable population:

If you are a scifi author and want to write an Amazons in space story, then you can use this world building premise without needing to cite me :D

Optional: Other XY + AY offspring patterns

As mentioned above the rules for XY + AY are not clear. It all depends on how a female Sex cell handles lacking a X chromosome. Is the X chromosome vital for the viability of the cell? or is it only vital for cell proliferation? which happens after merging with a sperm cell. Under all circumstances YY are not viable as the X chromosome contains a lot of important genes.

Let's game out all possible combinations

If egg cells containing a Y chromosome are viable from AY females then two other mating patterns are possible depending on what happens if a Y sperm tries to fuse with a Y egg cells.

  1. Y eggs 'reject' Y sperm Then:

    • AY donates A -> 50% XY + 50% AX
    • AY donates Y -> 100% XY
    • Thus 75% XY + 25% AX
  2. Y eggs 'accept' Y sperm, but the cell fails to divide

    • AY donates A -> 50% XY + 50% AX
    • AY donates Y -> 50% XY + 50% egg fails to divide
    • Thus 2/3 XY + 1/3 AX


12 comments, sorted by Click to highlight new comments since: Today at 6:28 PM
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1:10 was a good guess, but unfortunately the amazon gene only gets us to 1:3

It seems to me to make more sense to just go for artificial embryo selection. This especially goes for a spaceship with 10 people which is an amount that's a lot lower than what's generally considered to be necessary for a population to keep working.

Agreed, but then you don't get cool space amazons :). It could be an extra fail safe mechanism :)

You can get rid of men entirely and then have your space amazons.

Then maybe it only work for harem anime in space :)

This seems to be a very cool idea, but if (when?) we actually have the tech to do this, there would likely be better solutions to the issues this is supposed to help with. Or, our spacefaring capabilities would be such that the marginal advantage of Space Amazons would be very small.

Still I do hope someone makes this into a cool sci fi story.

Totally agree, it's also Christians critique of the idea :)... Maybe it could be relevant for aliens on a smaller planet as they could leave their planet more easily, and would thus be less advanced than us when we become space faring :)... Or a scifi where the different tech trees progress different, like stram punk

Wouldn't this prevent the A chromosome from having recombination with other A chromosomes? Which seems like a problem due to Muller's ratchet.

Good Point, In principle the X chromosome already has this issue when you get it from your farther, if the A chromosome is simply a normal X chromosome with an insertion of a set of proteins that blocks silencing, then you can still have recombination, if we assume the Amazon proteins are all located in the same LD region then mechanically everything is as in the post, but we do not have the Muller's ratchet problem

Also the A only recombines with X as AY is female and therefore never mates with an AX or AY

"Sci fi plot alert": what happens if due to random chance/genetic drift the "A" version of the X chromosome becomes more common and the last male dies?  This would be more probable the smaller the population is.  And, I dunno, a space rock snipes the sperm storage freezer.  (similar to what happened in Seveneves)

When the space ship lands there is a 1% chance that no males are among the first 16 births ()

Luckily males are firtile for longer so if the second generation had no men the first generation still works

If the A had a mutation such that AX did not have 50% chance of passing on a A, then the gender ratio would be even more extreme, if the last man dies the a AY female could probably artificially incriminate a female.

You can update the matrix and do the for product to see how those different rules pan out, if you have a specific ratio you want to try then I can calculate it for you, calculating a target gender ratio will require a mathematician as this is a Markov process and their Transmission Matrix are hard to calculate fom a target steady state, if you are a for mortal like me