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On my worlds of Earth, Ruquelis and Ersutiabu, there are three sexes relevant to this question, Men (XY male), Women (XX female) and Lilim (LX female), where X, Y and L are sex chromosomes. Lilim are effectively women with elongated, taloned feet and batlike wings with which they can fly (magically-assisted flight, but that magic isn't relevant to this question). Lilim- and women- children are indistinguishable until the age of 7-9 when lilim-children begin to grow wings and their feet begin to change into talons.

We therefore have crosses:

Men (XY) and Women (XX) -> 1/2 XY and 1/2 XX.

This corresponds to our familiar 50% male/female sex ratios in unions between men and women

However, we also have:

Men (XY) and Lilim (LX) -> 1/4 XL, 1/4 XX, 1/4 YL and 1/4 XY.

In these crosses, YL is immediately lethal, as the L chromosome is missing genes necessary for survival of a zygote that is only present on the X chromosome, preventing even a single round of cell division.

This leads to lilim effectively having a birth ratio of 1/3 XY Men, 1/3 XX Women and 1/3 XL Lilim, with a slightly lower birth rate than is the case for women.

In the distant past on Earth, lilim could not successfully bear XX offspring due to maternal/fetal incompatibility that caused XX woman offspring to be aborted in the first trimester, leading to a live birth ratio of 1/2 XY Men, 1/2 XL Lilim, and a significantly lower birth rate than is the case for women. Women also went extinct due to the higher survivability of lilim, until lilim were genetically engineered to eliminate the maternal/fetal incompatibility with XX offspring.

Since women reproduce more rapidly than Lilim, and breed true, this has led to undesirable social outcomes on other worlds on which populations of humans with lilim have settled. On Ruquelis, lilim prevent women from reproducing by killing or sterilizing girls who haven't begun to transform into a lilim by age 9, while on Ersutiabu, women are socially and economically discouraged from reproducing.

So, my question: How can lilim breed true once again?

I have considered putting an 'X-Polarase' gene on the L chromosome, so that during meiosis in a LX mother, any of the four resultant cells of meiosis that carry an X-chromosome will become a polar body rather than an ovum, but this would have the effect that lilim would then give birth only to lilim, as it would not be possible for them to supply the needed X-chromosome to an XY male offspring. This would lead to them becoming a parasitic sex, which is not what I have in mind.

I'd rather not go back down the road of maternal/fetal incompatibility between LX lilim mothers and XX woman offspring, so is there any other mechanism by which LX lilim and XY men can (almost) always have XY male and LX lilim offspring only?

In effect, I'm looking for a solution that maximises the LX lilim birth rate and minimising the XX woman birth rate from a LX lilim mother, rather than having fully half of their potential offspring being non-viable if at all possible. The genetic modifications need not be made to the lilim, but may instead be made to men, if necessary.

The best answer will involve the fewest gene changes and result in lilim giving birth to as few XX offspring as possible.

Additionally, since these changes must be distributed through the population by just one person who can only effect the change on one subject each ten minutes, for a maximum of 8 hours a day for a maximum of 1 month, it must propagate through the population naturally to as great an extent as possible.

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    $\begingroup$ So what is the goal here? To have more Lilim? Fewer XX females? A particular ratio of Lilim to XX births? To not have XX females predominate? They are actually quite different in solutions. In the past, XX females "died out" because of selective pressure. Why not allow selective pressure (like high teen death rates of XX females)? Also, first trimester losses mean a Lilim is ready to mate again quickly. If they have lower infant mortality/death in childbirth due to improved health, they may have a higher effective rate of reproduction. $\endgroup$
    – DWKraus
    Sep 26, 2022 at 15:52
  • $\begingroup$ @DWKraus, the goal is to have lilim effectively never give birth to XX women except perhaps for very rare exceptions, so that women need not be persecuted for their inability to bear lilim offspring. $\endgroup$
    – Monty Wild ♦
    Sep 27, 2022 at 2:58
  • $\begingroup$ Polysomy seems the solution, then hand wave a few things. Instead of L replacing a sex chromosome, it is an added chromosome. It doesn't even have to be in the 23rd set. $\endgroup$
    – fredsbend
    Sep 28, 2022 at 0:31
  • $\begingroup$ @frеdsbend, that would seem to complicate inheritance of lilim traits. $\endgroup$
    – Monty Wild ♦
    Sep 28, 2022 at 15:07

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Differential chemotaxis

Sperm use chemical gradients to reach the ovum. The receptors they use are genetically encoded, and so are the chemoattractant molecules produced by ova. You need to genetically modify both men and lilim, such that

  • male gametes carrying an X chromosome are more strongly chemoattracted to the lilim ova that have an L chromosome
  • male gametes carrying a Y chromosome are more strongly chemoattracted to the lilim ova that have an X chromosome

It's up to you which of these configuration uses the "default" chemoattractant pair and which you modify, but you will probably have to alter both the L and Y chromosome (we are probably trying to avoid modifying X because of the larger pool carried by women). The molecular mechanisms can be very varied - produce more of the existing chemoattractant from the ovum, increase/decrease receptor affinity on the sperm, produce a repellent molecule, alter surface linkage molecules used in the process of egg membrane breach, etc.

In this configuration, XX and YL zygotes are very rare, with the added bonus that lilim fertility increases (because fewer zygotes have the unviable YL combination).

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  • $\begingroup$ What happens in XX women? Won't that bias the birth ratio toward XY offspring? $\endgroup$
    – Monty Wild ♦
    Sep 27, 2022 at 3:20
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    $\begingroup$ Yes if it’s strictly gamete-dependent, but you could have the environment (lilim reproductive tissue) alter the chemoattractant gradient in a way that doesn’t happen in women, e.g. the X egg produces normal chemoattractant, but the lilim uterus produces something like neutralising antibodies that stick to X sperm only and “blind” them to it; this doesn’t happen in women, so X and Y sperm are equally attracted. L ovum attraction is fine because it uses a different chemoattractant, and here the bias is imposed by e.g. secreting a repellent for Y. $\endgroup$
    – Ottie
    Sep 27, 2022 at 7:06
  • $\begingroup$ @MontyWild That sounds like a free bonus, not a drawback! The people doing this modification are trying to minimize the XX birth rate, and every XY child of an XX woman is a child that would otherwise have been another XX woman. If women predominantly give birth to only men, then the problem will solve itself naturally with no social pressure. $\endgroup$
    – Douglas
    Sep 27, 2022 at 23:06
  • $\begingroup$ @Douglas that's a good point, and in any case lilim birthing 2/3 female offspring would have skewed the population already, so a correction in favour of males might be desirable anyway. Eventually, however, you'd probably want parity between whatever compatible sexes you have left. $\endgroup$
    – Ottie
    Sep 28, 2022 at 12:51
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Lilims twin a lot

Fertilised lilim ova have a far higher chance of twinning than humans. Around 50%. (Maybe 100%?). Even triplets are common, and quadruplets are only as uncommon as human triplets.

This not only boosts their birth rate substantially, but tends to provide the lilim with at least one highly loyal sibling, and, if things go badly, an aunt for their children.

This has some major effects on lilim society and family life. You can create conventions/family structures for a society where twinship is common or even nearly universal. Drawing on books by or about twins will yield loads of ideas you can riff on. You can decide if the twins are fraternal or identical. Edit: On second thoughts, only identical twinning really helps. You get two LX offspring per fertilisation.

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  • $\begingroup$ This doesn't give a mechanism by which lilim can breed true and not bear XX offspring. Furthermore, carrying multiple offspring makes even less sense for a flying female than it does for a non-flying one. $\endgroup$
    – Monty Wild ♦
    Sep 26, 2022 at 8:52
  • $\begingroup$ Not 100% true, no, but it gets you far closer. And the sheer demographic advantage would massively outweigh a slightly heavier body during pregnancy, flying or non flying. Make their hips wider, maybe? And it does involve almost no gene changes and maximises birth rate (both absolute and lilim specific), as requested. It's your story though :) $\endgroup$
    – AncientGiantPottedPlant
    Sep 26, 2022 at 8:56
  • $\begingroup$ Uh... the whole point of the question was to get answers to the question, What mechanism can best prevent lilim from conceiving XX children? Your answer won't alter the sex ratio at all. $\endgroup$
    – Monty Wild ♦
    Sep 26, 2022 at 9:04
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    $\begingroup$ @MontyWild Normally, yes, flying creatures would have few to one offspring at a time; bats only have one child at a time, and for their size gestation and time to maturity are extremely long. However, you've already got flying humans that you note use magic to assist their wings; it's not hard to stipulate in your world that the magic can take another few pounds of extra weight in a twin pregnancy. $\endgroup$
    – Palarran
    Sep 26, 2022 at 9:06
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    $\begingroup$ You asked for "breeding true", not "What mechanism can best prevent lilim from conceiving XX children?" Breeding true means producing lilim. Which twinning does move towards. After all: "In effect, I'm looking for a solution that maximises the lilim birth rate, rather than having fully half of their potential offspring being non-viable if at all possible. ". Sure, it's not the answer you want, no problem. It's not a stupid answer or at odds with the way you've actually phrased the question. Lets stop litigating it. Have a nice day. $\endgroup$
    – AncientGiantPottedPlant
    Sep 26, 2022 at 9:11
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Chimeras:

Your Lilim have a modified form of meiosis. Lilim eggs don’t separate in meiosis. So there is an L egg and an X egg together. The X egg is the one fertilized. If the sperm carry a Y, the L egg fails to survive and the Lilim gives birth to a male. If the sperm carry an X, the XX egg undergoes a limited form of meiosis to make a copy of the DNA and transfer it to the other egg so the L egg becomes an LX.

The organs critical for life derive from the XX twin, while the LX twin gives rise to the reproductive tissues an those that will be responsible for the Lilim phenotype. At the appropriate developmental stage, the LX tissues begin to replace certain XX tissues to create the Lilim body plan.

Enough things go wrong in this process to give rise to a limited number of XX females without the LX chimeric twin. That percentage can be whatever you want for your story.

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  • $\begingroup$ I'm not sure that I understand L and X eggs not separating during meiosis... are you saying that both an X and a L become eggs, or that the two gain the shell of a single egg? Wouldn't that require either double fertilization or cause ploidy problems? $\endgroup$
    – Monty Wild ♦
    Sep 27, 2022 at 3:09
  • $\begingroup$ @MontyWild Normally in meiosis, the pre-egg divides twice and three of the four cells are discarded and do not develop into an egg. In this case, the final division (resulting in haploid cells) does not discard the additional cell and instead they remain connected. As long as we are messing with meiosis, I have the Sperm DNA replicating an additional time (methylation regulated?) so the ploidy works out. It's a lot of steps where something can go wrong, and the only likely viable mistake would result in an XX female resulting. So you can recover XX females, but get mostly XX-LX chimeras. $\endgroup$
    – DWKraus
    Sep 27, 2022 at 4:32
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    $\begingroup$ I think this is a beautiful answer and fits with other aspects of the lore, but I’d have no idea how to implement it genetically… $\endgroup$
    – Ottie
    Sep 27, 2022 at 7:08
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Uterus bacteria

Create or preferably alter some bacteria that live in the uterus. These do their normal symbiotic tasks. However, the bacteria are triggered when anything other than a LX has nested. During the pregnancy the different hormones exuded from non LX fetus makes the bacteria react. They turn the environment inhospitable for any fetus. If the child is LX there won't be a reaction.

The person delivering this only needs to insert the bacteria colony into the uterus. For spreading it effectively to everyone you can use the same way many good and bad bacteria spread. HPV, AIDS, gonorrhoea are easy examples. Lilim can assist the spread by engaging in certain sexual activities. The bacteria can then spread from person to person.

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  • $\begingroup$ How about XY? Men are needed too. Also, this would be transmitted to XX women from men, making them infertile. $\endgroup$
    – Monty Wild ♦
    Sep 27, 2022 at 3:03
  • $\begingroup$ @MontyWild XY can be added to the list where it doesn't react to. Either it reacts to a specific XX hormone, or stays calm due to specific LX/XY hormones. XX women becoming infertile or only able to give birth to XY can help your cause. Though you could give out medicine to suppress the reaction. As the bacteria isn't spread on purpose to XX woman we can assume it isn't widespread in their population. It can also be more difficult for men to receive and transmit the bacteria. And just like in real life STD's are just in a tiny subset of the population. $\endgroup$
    – Trioxidane
    Sep 27, 2022 at 9:26
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A simple solution

The lilim's X(human female?) chromosome in their gametes tend to become human Y chromosomes, making it possible for them to produce L, X, and Y egg cells. This leads to the possibility of LX, XY, XX, LY, and YY zygotes.

As you've stated, LY is inviable. I don't know what would come from a YY(as the above change would make that possible, probably somewhat likely) but let's make that inviable as well. This leads to the only viable obtions being LX, XY, and XX. Since the lilim X chromosome only sometimes doesn't turn into a Y due to [insert your reason here], the vast majority of lilim offspring will be either human males or lilim, with about 40% of their fertilizations being inviable.

Edit: I see that some genetic changes must be introduced throughout the population to make it so that quite literally only lilim or human males are born. In this case the solution would be to find what causes the change to sometimes not occur and then make changes so that the X to Y change always occurs(barring genetic mishaps and such).

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  • $\begingroup$ The Y chromosome is much smaller than the X chromosome, and is just similar enough around the centromere that it pairs with the X during cell division. It also carries some unique y-only genes. An X chromosome cannot simply turn into a Y. $\endgroup$
    – Monty Wild ♦
    Sep 27, 2022 at 3:05
  • $\begingroup$ @MontyWild Magic exists in your setting so without an understanding of your magic really any kind of bibidi babidi boo answer should satisfy your question. How you've set them up make it really difficult not to just use a magical solution. $\endgroup$
    – Lemming
    Sep 27, 2022 at 15:32
  • $\begingroup$ @MontyWild However, I will offer the alternative that perhaps it could be made that depending on the chromosome of the egg cell present the lilim womb is highly inhospitable specifically for either X or Y-bearing sperm cells. If the ovum present is L, then it's inhospitable for Y, if it is X then it's inhospitable for X. $\endgroup$
    – Lemming
    Sep 27, 2022 at 15:58
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I'm throwing my latest idea out to compete with the rest of the ideas here...

Mitochondrial incompatibility

In humans, both eggs and sperm carry mitochondria, but the sperm's mitochondria are not inherited, as they do not enter the egg during fertilization. They are of matrilineal descent.

Secondly, X, Y and L chromosomes each have genes that are common to all three chromosomes, genes common to two but not the third (X and Y but not L, X and L but not Y, & Y and L but not X), and unique genes not shared by either of the other two chromosomes.

So, by modifying the mitochondria to add a gene lethal in diploid cells (preventing cell division) that is inactivated by a protein that is produced by a gene on both the Y and L chromosomes, but not on the X chromosome, XX cells will not divide, but XY or LX will. YL will still be lethal by its own mechanism.

Lilim mothers with the modified mitochondria will therefore only give birth to LX lilim and XY men. These mitochondria will be inherited by all of the lilim's offspring, but will be transmitted only by the female offspring, so it will not make its way into XX women.

Lilim birth rates will remain slightly lower than women's, but as pregnancy with XX (or YL) offspring will not occur, the probability of pregnancy per menstrual cycle in lilim will be half of that of women. Unlike the former maternal/fetal incompatibility, XX offspring won't result in a pregnancy that is miscarried in the first trimester.

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The Lilim's gamete selects for the "correct" sperm arriving

Contrary to popular belief, it's not the first sperm arriving at the egg that fertilizes it, but usually one of the almost-first ones in a (currently) hardly understood process. However, some selection is involved (in humans) since even with a 50:50 sperm sex ratio, the fertilized eggs are NOT evenly split, to account for more XY embryos not making it through pregnancy and early childhood.

So while absolute pure-breeding would be hard to explain with this mechanism, a significant skew towards LX offspring could be the case, enough for a stable Lilim population to exist.

Alternatively: maybe the reproductive system even flushes out (almost) all non-L-carrying eggs, meaning that it's either nothing (if a Y sperm makes it), or a lilim if an X-Sperm makes it. The occasional XY and XX births from Lilims are causes of this selection not working absolutely and can be as rare/frequent as your story/world requires.

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Not quite what you asked for, but you could disrupt non-lilim breeding using a gene drive. Modify the L and X chromosomes to carry a payload that hijacks DNA repair mechanisms to copy itself over to unmodified L and X chromosomes, resulting in new chromosomes with the same payload ("correcting" X chromosomes in XX' progeny or L chromosomes in LX' to make them X'X' and L'X'). The key part: men with X' chromosomes make sperm that are viable only in lilim females.

  • XY + XX -> 2 XY, 2 XX
  • XY + L'X' -> X'X', L'X', X'Y
  • XY + X'X' -> 2 X'Y, 2 X'X'
  • X'Y + L'X' -> same as XY + L'X'
  • X'Y + LX -> same as XY + L'X'
  • X'Y + XX -> sterile
  • X'Y + X'X' -> sterile

Then in subsequent generations, X'Y males can only reproduce with lilim, and X'X' females are only fertile with XY males but produce only X'X' females and X'Y males. The modified chromosomes can only be removed from the gene pool by preventing every individual carrying them from breeding, and without intervention, the unmodified X chromosome becomes extinct in a relatively small number of generations.

This doesn't make lilim "breed true" (unless you make the X'X' combination non-viable, which you said was undesirable), they are still equally likely to produce a "normal human" daughter or a lilim daughter, but they become the only fertile females: in the end, all humans have lilim mothers, and it becomes impossible to have a self-sustaining population without lilim.

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