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Humans and many mammals have an XY sex determination system. This means that the presence of a Y chromosome makes an individual male, while having 2 X chomosomes (and lacking a Y chomosome) makes an individual female. Birds and other species have a ZW sex determination system, where the presence of a W chromosome makes an individual female.

In my Gyre world, I have used the XY sex determination system to add sexual morphs. Male morphs are easy: Unicorns have an X and a U chromosome. Functionally the U chromosome is a Y chromosome, except that it is larger than the X chromosome, and it carries the sex-determining SRY gene, along with other genes that make the child carrying it develop into a unicorn in-utero. Wyrms are similar, having an X and a D chromosome.

This works out 'easily' since sex determination with normal men is:

XX x XY = ½ XX, ½ XY

Substitute Y with U or D, and it still works. Of course, unicorns and wyrms must mate with women in order to reproduce, but the gender balance is preserved. Unicorns and wyrms are simply male morphs. In the ZW sex determination system, female morphs would be 'easy' to implement by replacing the W chromosome, as females are ZW while males are ZZ.

However, when it comes to chromosomally inherited female sexual morphs in humans, it is not so easy. To use the example of lilim, which are women who have batlike wings and taloned feet, lilim have X and L chromosomes, where the L chromosome functions similarly to an X chromosome. This lack of ease would also apply to males in the ZW determination system. The genetics work out as:

XL x XY = ¼ XX, ¼ XY, ¼ XL, ¼ LY

So, we can see that we have an unwanted LY offspring... what can we do with it? Make LY a lethal result? Make it into yet another male morph or have it be functionally identical to XY males, which would further complicate the genetics:

XX x LY = ½ XL, ½ XY

XL x LY = ¼ XL, ¼ XY, ¼ LL, ¼ LY

From which we get LL, which is an additional complication:

LL x XY = ½LX, ½LY

LL x LY = ½LL, ½LY

So, if we also allow LY males and homozygous lilim, we end up with lilim only sometimes breeding true, and sometimes seeming to skip a generation... which is fine as long as that's what you want, but I didn't want that.

In my Lilim, I went with making LY lethal, though LXY, which involves a rare error during the process of meiosis, results in an infertile male lilim. However, this means that the viable offspring then become 1/3 female, 1/3 male and 1/3 lilim. Since women and lilim are female, this upsets the 50% gender balance and makes lilim less fertile than women, and unless lilim are more desirable as mates than women, will lead to extinction of the morph.

So, after all that background, is there any way that I can have an 'easy' genetically-based female-only morph where the female morph reproduces only itself 50% or 50% normal non-carrier males, where the morph can not skip generations?

It need not be a sex-chromosome-based system like the lilim, but I would prefer some sort of chromosome-based system where the morph is carried on the chromosome.

This question is similar to How to get the Lilim to breed true?, but differs in that I'm asking about any way that a female morph can breed true, rather than only the ways that lilim can be modified to breed true.

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    $\begingroup$ On a completely irrelevant side issue, if I read your Q correctly, a human female can bear a unicorn? Ouch! Hooves!Baby kicks! What do you mean you have to turn the baby? $\endgroup$
    – JBH
    Nov 21, 2022 at 4:00
  • $\begingroup$ @JBH Correct... That's actually something that my story will go into. It will be easier for a woman to give birth to a baby unicorn than to a normal baby, due to the relative length and narrowness of its head compared with a normal baby's. As for hooves, hoofed animals don't have a problem, the hooves have a soft pad that comes off after birth that prevents injury to the mother, and the horn grows after birth. $\endgroup$
    – Monty Wild ♦
    Nov 21, 2022 at 12:37
  • $\begingroup$ I don't think any sex chromosome-based system will achieve what you want. The easiest way to get the 50-50 ratio is to have the lilim morph caused by an autosomal dominant gene that is activated only in females. With a sex chromosome-based system, you'd get closer by making the L trait dominant over X but not Y, so that LL and LX would both be lilim. LX lilim would still have a 25% chance of having an XX daughter if their partner is XY, but maybe LY men could somehow be more attractive to lilim than XY men. (In the long run, that would eventually cause a split into two species.) $\endgroup$
    – Cloudberry
    Nov 21, 2022 at 20:34
  • $\begingroup$ @Cloudberry, It need not be a sex-chromosome-based system $\endgroup$
    – Monty Wild ♦
    Nov 21, 2022 at 21:40

3 Answers 3

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Driver chromosome.

I have a recycled answer for you here, Wild. Maybe your question is a duplicate of this one?

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

The recycling:

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

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!.

In the Lilim scenario, X! is called L. It is an altered X. It is a driver, and ensures all ova in Lilim are L by suppressing the X partner during meiosis. Lilim only make L ova.

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  • $\begingroup$ How do you allow for the male offspring of an X! morph not passing the X! chromosome to his female offspring? I need X!X females and XY males, not X!Y males. $\endgroup$
    – Monty Wild ♦
    Nov 21, 2022 at 12:22
  • $\begingroup$ Maybe some of your X!Y females are phenotypically male. $\endgroup$
    – Willk
    Nov 21, 2022 at 13:16
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LY is female.

The L chromosome is dominant, and LY has a female phenotype. Crosses between LY and XY yield LX, YY, XY and LY. YY is nonviable. LX is also nonviable. Thus 50% of pregnancies in LY females miscarry early; so early that the impact on fitness is negligible. Of viable pregnancies they are half Lilim LY and half male XY.

Once could compensate for the sex chromosome related 50% miscarriage rate by having LY females with a propensity to conceive twins with a 50% chance of one of the twins miscarrying and a 25% of both miscarrying, but a 25% chance of viable twins. The viable conception rate for Lilim is thus the same as XX females.

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  • $\begingroup$ This is still messy. Y is the sex determination chromosome, so how would you suppress the SRY gene? $\endgroup$
    – Monty Wild ♦
    Nov 24, 2022 at 13:11
  • $\begingroup$ The L chromosome has a copy of SRY and so unlike XY males LY females have 2 copies. There are reported cases of XX males where SRY occurs on X as a meiotic error - so not that farfetched. You could make the L copy of SRY autosomally dominant, suppressing the Y copy. This would be the simplest. $\endgroup$
    – Willk
    Nov 25, 2022 at 17:52
  • $\begingroup$ So, that makes lilim that are supposed to be female into males, or is there more to it? $\endgroup$
    – Monty Wild ♦
    Nov 26, 2022 at 14:49
  • $\begingroup$ No, they are just how you describe them. Bat ladies. The L is mostly an X chromosome but more - genes for wings and Lilim things, the rest of the X, and the dominant negative SRY. $\endgroup$
    – Willk
    Nov 26, 2022 at 16:05
  • $\begingroup$ The reason I envisioned the LX to be nonviable is because of lyonizaton of the L and so problems in the embryo with patchy expression of the Lilim specific genes. $\endgroup$
    – Willk
    Nov 26, 2022 at 16:06
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I finally worked this one out myself...

We have three sex chromosomes, X, Y and Z. The Z chromosome carries the genes for the female-only morph. X is the regular female sex chromosome. Y is the male sex chromosome.

My problem is that I want a system where the presence of a Y-chromosome means that there must be a maternal X chromosome, while the presence of a Z-chromosome means that there must be a paternal X chromosome. However:

ZX x XY -> ¼ZX, ¼ZY, ¼XX, ¼XY

Fully a half of these offspring are undesirable, one quarter being ZY 'male morph' types that I don't want to have exist at all, and the other quarter being XX women that mean that the female-only morph isn't breeding true.

My problem has been how to exclude these. For male-only morphs, it's easy, just change Y to something else that carries the morph. For females, I've shown how it's harder. The solutions I've seen are all messy, involving driver chromosomes, non-viability, mitochondrial incompatibility, and others that reduce reproductive fitness.

However, the solution seems to be to get a newly fertilised single-celled zygote to do the choosing for itself.

This involves the Z chromosome causing a slight change to the process of meiosis, where nondisjunction of X and Z (only) is forced, so that instead of producing 2 gametes with X and two with Z, there are two with XZ and two with no sex chromosomes at all (-). One XZ oocyte and the two - oocytes become non-viable polar bodies. So, we then get:

ZX¹ x X²Y -> ½ZX¹X², ½ZX¹Y. Note that I've distinguished the maternal X as X¹ and the paternal X as X².

So, in the single-celled zygote, we have another, additional process to correct the nondisjoined XZ chromosome pair.

By producing a SRY detecting protein (SRYDP) protein that selectively binds to the SRY gene, and alters its shape (unbinding from a nondisjoined X-chromosome if it is already bound to one), and can then bind to a nondisjoined Z-chromosome, we can use that as a starter marker for a selective DNAse that destroys the unneeded maternal Z-chromosome.

Alternatively, if there is no SRY gene for SRYDP to bind to, its shape is not altered, and it binds to a nondisjoined X chromosome, that is then used as the starter marker for the selective DNAse to destroy the unneeded maternal X chromosome.

So, after this process we have:

ZX¹ x X²Y -> ½ZX¹X², ½ZX¹Y -> ½ZX², ½X¹Y

Which results in 50% of the offspring being the desired true-breeding female-only sexual morph, and 50% males.

Failure of non-disjunction would result in the occasional birth of an XX woman from a ZX mother, and the occasional non-viable YZ offspring.

Failure of the nondisjunction correction process would result in an occasional ZXX morph-form female equivalent to a trisomy-X woman, and an occasional XYZ morph-form infertile male, similar to a Klinefelter syndrome male.

These errors ocurring occasionally are acceptable.

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