I had an idea about a lineage supposedly cursed so that the women only give birth to more women, but I'm thinking that some outside force purposefully mutated/altered their DNA to cause the effect without their knowledge. What mechanism might be used in such a scenario to prevent any male children from being born, or maybe even conceived?
Its been observed that jet-pilots tend to have more girls than boys. Its even something of an inside joke among aviation types. Its been given some scientific credence lately when it was theorised that higher exposure to cosmic radiation or potentially even the high G-forces could be damaging their Y chromosome. Due to a quirk in chemistry the composition of the Y chromosome is slightly less stable than that of the X chromosome. If exposed to radiation or other environmental contaminants or factors at the right level it can potentially not be a high enough exposure to create full sterility, but will still damage the Y chromosome in the ahem "awaiting swimmers." This could greatly increase likleyhood of female offspring.
Perhaps there is some malign force targeting the men these women are with. It's sinister because trying to figure out why the women are only having daughters probably wouldn't look for a source that's actually targeting their husbands. Could make for a good plot twist.
Self produced testosterone antibodies.
As soon as the male embryo starts producing testosterone, it gets attacked by the mother's antibodies and as a result cannot complete its development.
This is an extreme case of the so-called Mother's curse:
In biology, mother's curse is an evolutionary effect that males inherit deleterious mitochondrial genome (mtDNA) mutations from their mother, while those mutations are beneficial, neutral or less deleterious to females.
There are known cases where an mtDNA defect largely or solely affects males of the species (e.g. in humans, Leber's hereditary optic neuropathy). All you need is an mtDNA defect so nasty that it renders male embryos non-viable, rather than merely predisposing them to blindness.
This works despite the limitations of mtDNA itself; it is known that, at least in fruit flies, mtDNA polymorphism can affect nuclear (DNA) gene expression in males without affecting females. Even though mtDNA doesn't do a lot on its own, it can turn on or off critical parts of the nuclear DNA, causing male embryos to die, or (for a weirder "solution" to the problem) completely deactivate the Y chromosome, causing them to develop as females.
As is, there is a theory that part of the lower life expectancy for men is that there are tons of small mother's curses lying around the human genome; since a mother's curse is harmless to women who have it, and the men who have it aren't responsible for propagating mtDNA, from the mtDNA's point of view, there is very little selective pressure to correct the problem.
That said, Mother's curses don't tend to spread much on their own; there is little pressure against them, but neither is there (usually) much pressure that spreads them. A founder effect, combined with exclusionary policies on breeding (no outside women may join the community, but outside men are encouraged to do so) could create this sort of scenario. Alternatively, a mother's curse that is actually beneficial to the women who have it (increased fertility, more efficient metabolism, what have you) might create such a community naturally, but it would be inherently unstable; such strong selective pressure would eventually lead to extinction, as more and more women came to possess a gene that prevented conception of men, leaving few men available for breeding at all.
This actually happens in insects (and other invertebrates) via the bacterium Wolbachia. I suggest you research its male killing effects and borrow from there, Wikipedia is a decent place to start. There's no direct mammal parallel to these effects (that we know of) but it should provide a source of face-valid ideas.
In general, genetic effects will work poorly for a lineage, because recombination means that only half of the genes are passed on each time. As a result, it would be diluted as the generations pass rather than persistent. You need a mechanism to explain why this does not happen as well as well as a mechanism to explain how it has its effect.
Consider: there are conditions that are passed on by X-linked recessive inheritance. The most famous is haemophilia. Here's how it works: mutation BadNews is recessive, and carried on the X chromosome. The mother is a carrier. A daughter would get a healthy X chromosome from the father, and either a healthy or a mutated X chromosome from the mother - she is either unaffected, or a carrier - healthy either way. The son only gets one copy of the X chromosome, from his mother, so he is either healthy, or sick.
Now, imagine that mutation BadNews is so lethal, that the foetus fails to develop, resulting in a miscarriage. Mutations that are so lethal exist, and they could conceivably be X-linked. It's possible that there is an actual disorder that would fit those requirements.
But that still leaves the possibility of a healthy boy, you say. 50% of boys conceived would be healthy, resulting in a 2:1 girl:boy ratio. True. There are several ways you can address that, depending on the way you plan your story.
- The family is aware of the condition, and having to resort to IVF for unrelated reasons, pick only female foetuses. (Picking only females is much easier and cheaper than picking healthy males).
- A really unlikely but theoretically possible situation: the mother carries not one, but two different X-linked recessive lethal conditions: BadNews and TerribleNews, one on each X chromosome. The boys get either one and die, the girls get either one + a healthy X chromosome from the father.
- The family could have in fact had a healthy son, but with chances being 2:1 in favour of a girl, the dice just fell this way.
Easiest, biological explanation, spermatozoid with Y chromosome are more vulnerable to acidic pH in women vagina. So a lineage may mean that women living in certain area have that ailment due to environment (lack of sodium and potassium in diet while water used for bathing have acidic pH)
A fantasy somewhat setting based on a little of anthropology and old wives tales.
In Western Europe there is a saying/belief that when there number of boys birth rise it mean the war is brewing. It's based on the fact that during the war the men are the ones who dies so someone would need to replace them and fill that gap that will be done to population of able men.
Marvin Harris in his Cows, Pigs, Wars, and Witches explain why A) female cows are untouchable in India (because they give birth to oxes that do all the work and die frequently so one cow can during her life can breed many oxes) b)why native on New Guinea have Men housing and wars (called Keiko) are fought by rising and slaughtering pigs every two-three years (because it's better to kill pig every 3 year than a men who have other purposes and abilities)
So back to premise: why there would be no need for boys to be born:
- Men and women are equal for a long time so there is no need to "save" women from war or hard labour (imagine Amazons)
- the quality of life is so high that there is no biological need for men to be born (there is no men specific tasks)
- women decided long time ago they don't need local man and can go wherever they please and mate with whomever they please and their bodies over time adapted to fact that there is no such think as "insufficiency" of men and it's better to breed girls as they can give birth to much more humans than any men will ever do.
There's some prior art in nature, sadly. If we look at recurrent miscarriages, one of the causes is male-specific minor histocompatibility. In this a mother develops an immune response to male-specific antigens, which causes miscarriages. It primarily occurs after giving birth to a first-born boy, who primes the mother's immune system just like how a first bee sting primes the immune system of an allergic individual such that the second is deadly. The result is that we see a dramatic change in sex distribution from the firstborn to the others (which are predominantly girls).
If you were to extend this further, you might be able to develop a reason why this effect gets stronger and appears before the firstborn instead of after. The result would be a very strong tendency to miscarry boys.
You're looking for a line of women who only have daughters, and whose daughters always share this trait.
This is doable. What I think may be most reliable way to pull it off requires some hefty genetic engineering, but nothing beyond what current technology is capable of.
The trick is to use a gene drive.
Pick a spot between two base pairs in an unused location in one of your target's chromosomes (doesn't really matter which one; a body chromosome might give slightly better results than an X chromosome, for reasons I'll explain later), and fabricate a strand of DNA containing the following:
- A copy of the nucleotides on one side of the chosen location
- A nucleotide sequence encoding the Cas9 enzyme, which, when given a strand of RNA as a guide, will scan a cell's DNA for a section that matches the RNA guide, and cut the DNA right there
- A copy of the base pairs immediately around the chosen location on both sides, which will be transcribed into the guide RNA for the Cas9
- A gene that will cause the cell to self-destruct in the presence of a Y chromosome. Maybe a gene that is inactive by default, but is activated by the protein produced by the SRY gene (which is located on the Y chromosome and causes maleness), and triggers programmed cell death. Or something like that. It doesn't really matter, as long as the presence of this gene and a Y chromosome in the same cell causes the cell to die
- And, finally, a copy of the nucleotides on the other side of the chosen location
When this strand of DNA is inserted into a cell, the following happens:
- RNA Polymerase enzymes in the cell transcribe the DNA into RNA, and then ribosomes read the RNA and synthesize the Cas9 enzyme
- The Cas9 programs itself with the guide RNA, then diffuses its way into the nucleus of the cell
- The Cas9 sifts through the cell's DNA until it finds a section that matches the guide RNA- which will be the location chosen earlier
- The Cas9 cuts the DNA strand right there
- The cell's built-in DNA-repairing mechanisms see that one of the chromosomes has been cut, and looks around in nearby DNA for the same sequences as the cut ends of the chromosome (which would normally be the corresponding point on its paired counterpart)
- The DNA-repairing mechanisms find that the original DNA sequence you fabricated and inserted into the cell fits perfectly, and "repair" the chromosome by copying your DNA to fill the gap
In effect, you've created a strand of DNA that inserts itself into a predetermined location in a cell's genome, in both members of a chromosome pair. Permanently. Get this into a woman's ovaries (maybe by way of in vitro fertilization, maybe after she's grown up by way of an engineered retrovirus), and the curse is laid.
Whenever a woman carrying this "curse" conceives, the Cas9 will immediately go to work in cutting the appropriate chromosome from the sperm in the same, preselected place, and the zygote's DNA-repair machinery will go to work copying over the "curse" DNA from the egg to fill in the gap, by exactly the same means described above. Thus, a cursed woman's children are guaranteed to carry two copies of the "curse" gene. Then, if the sperm carried a Y chromosome, the self-destruction part of the "curse" will trigger, killing the embryo and causing a miscarriage long before the woman could realize she was pregnant. However, if the sperm did not have a Y chromosome, the self-destruct gene will not trigger, and the woman will bear a viable daughter.
I mentioned earlier that it's probably preferable if this "curse" gene inserts itself into a body chromosome, rather than the X chromosome. This is because, if the "curse" is in a body chromosome, all of a "cursed" woman's descendants are guaranteed to carry two copies of the "curse". If, by some fluke, she manages to have a son, the son's children will also inherit the "curse", regardless of their gender, so he too will only have daughters. If, on the other hand, the "curse" was aimed at the X chromosome, the improbable son would only have one copy of the "curse" (because he has only one X chromosome). He would be able to have sons and daughters like any healthy man (because his sons would not inherit his "cursed" X chromosome), and though his daughters would still carry the "curse", his sons would be free of it entirely.
Come to think of it, that could actually make for an interesting plot. Whatever. It's up to you. Have fun with it.
If the females are genetically modified to use parthenogenesis for reproduction, all their children would be daughters.
The daughters would also have very little genetic diversity. They would get all their genetic material from their mothers. There's a fifty percent chance that they would get the same chromosome for any specific instance. Then what that woman became a mother, she would only be able to provide that chromosome. After around six generations or so, all the chromosomes would be doubled in any particular mother. From that point on, the daughters will be essentially clones of their mothers.
It's unclear what would trigger reproduction. Would it happen after a fixed period of time? In reaction to (not as a result of) sex?
How many daughters would they have? As many (or few) as they wanted? A fixed number? The method of triggering reproduction would probably affect this as well.
This would have considerable side effects that may make it unsuitable for your story. For example,
- None of the daughters would have fathers genetically. They would get all their genetic material from their mothers. Once the genetic material is all doubled (all twenty-three chromosome pairs are of identical chromosomes), the daughters would be essentially clones of the mothers.
- There would be a high number of double recessives appearing, so some lines may not be viable.
- Condoms might not prevent pregnancy, which does not require men's genetic material.
- Since men are unnecessary for reproduction, they might have an uncommonly high number of single mothers.
You already got a few methods for filtering out male embryos, but keep in mind that genes have a 50% chance of being inherited (except when the parent is homozygous, but then the next generation is heterozygous). So you need a mechanism that is inherited 100% of the time.
If you wanted a male only having male offspring, you could work with the Y chromosome, but for females this is not possible. I can see two ways to achieve this for females:
1) You could work with mitochondrial DNA which is always 100% inherited from the mother, but this limits your options what the genes could do.
2) You need a retro-virus that is somehow transferred from the mother to the embryo. However, infecting the child is not as easy as it may sound (umbilical cord and amniotic fluid are not good infection vectors). The possibilities here depend on the medical knowledge of you characters. Maybe the virus modifies a gene that kills male embryos (see other answers) and is then transferred via breast feeding to the child, but this is not known to the family.
Sex selective mutations already exist, in mice there is a famous one the R2D2 mutation (yes that is really the name) prevents female offspring (X gamete fail to develop), Since the gene is a Y gene all offspring will carry it.
Now I know you want all female but a similar X linked gene that favors only female offspring is completely possible. Such genes are called SA-zygotic drives There is a whole list of mechanism by which they can operate.
One of my favorite is a pair of linked genes on the X chromosome that produce a toxin and an immunity to said toxin, Any gamete carrying a Y lack the defense against the toxin and dies but the X gametes and the parent cells are fine.
Climate change is already causing an increase in the relative death rate of male fetuses: https://www.smithsonianmag.com/smart-news/could-climate-change-affect-number-boys-and-girls-born-180952935/ (Well, maybe not climate change, but some stress factor. More research is ongoing.)
Males are just more fragile than females.
Invent some toxin that kills 50% of female fetuses and 99.9999% of male fetuses.
(I read an SF story once about a future Earth where all the men had died off and women reproduced parthenogenetically; when someone revived a Y chromosome from ancient DNA and showed the adult male to scientists, the protagonist's first thought was "the ugliest woman I have ever seen". But you are not creating an all female society, just a lineage blessed with all daughters.)
In your cursed family, babies are rare, precious, and overwhelmingly female.
I have a crayfish in my aquarium that is a mutation of a common species that originates in Florida. All members of her species are female, clones of the same original crayfish. The mutation ensures that meiosis does not occur, and that the eggs produced grow clones of the original mother.
I had a friend in college who was the 6th generation of female only children on her mothers side. Now assuming each woman had only one pregnancy that's 1/64 (2^6) (as was the case for my friend). It depends on how long you want the lineage to be. Having the child at 25 years old means that is 150 years of female-only offspring.
Increasing the number of children per mother makes the odds get long very fast. The odds of 2 female children per generation for 6 generations becomes 2^12=1/4096 which again isn't all that unlikely.
Now 7th daughter of the 7th daughter of the 7th daughter with all female offspring? Minimum number of females is 22 (only the Mother and Grandmother had 7 daughters each, no other offspring). Maximum number if every female had 7 daughters = 1+7+49+343 = 400. Odds? 1 in 2^399 so at this point you need some biological help.
If you allow gene therapy, I like Someone Else 37's answer based on a gene drive, mainly because it sounds plausible and most stories/worlds don't need 100% achievable today tech. The initial female (Eve) probably needs to be built in a test tube and artificially inseminated rather than delivering the genes with a retrovirus.
Alternatively, assuming you do not want external influence over the male father's sperm, you need to make reproduction hostile to either the presence of the Y chromosome or a male foetus. That way the fertilisation of male children is possible but none of them come to term. Ideally they miscarry in the first trimester so it's not as big a deal given quick internet search says between 10 and 25 of known pregnancies end in a miscarriage.
There is a form of reproduction called Gynogenesis in which the reproductive process of the female requires sexual intercourse with a male but none of their genetic material is incorporated into the offspring. The example I am familiar with is the Amazon Molly, Poecilia formosa (also called Mollienesia formosa, as in the link above) which reproduces by mating with males one of the co-occurring species such as P. latipinna or P. mexicana and is thought likely to have original arisen by a hybridisation event between these two species.
Because the eggs still undergo meiosis and recombination events occur, daughters are not direct clones of their mothers, which would introduce some variation but also a substantially higher degree of genetic problems as with extreme examples of inbreeding. There would likely also be an increased number of miscarriages from the same problem.
Mechanistically this process would require a change to the development of eggs in the oocytes so that they remain diploid - which seems a plausible mutation since it is something that occurs at low rates in most women anyway - and some mechanism to prevent the genetic material from the sperm joining the egg which also seems fairly plausible. Obviously extremely advanced genetic manipulation techniques are required for this and the mutation must be homozygous.
Because there is no genetic transfer it gets round the objection from my other answer that the mechanism would be diluted by recombination.
A mutation in mitochondrial DNA that causes the mitochondria to shut down or produce a lethal toxin in the presence of one of the following:
- typical male levels of testosterone
- Y chromosome (if the mitochondria ever have proximity to it?)
- some RNA or protein produced from the Y chromosome
Unlike mutations in the mother's own chromosomes, which would not survive as a lineage (as in Galastel's answer), mutations in mitochondrial DNA will remain until they mutate again or the lineage dies off.
It could be a plan to wipe out all humans.
There is a plan to eradicate species of mosquito by genetically altering them to only produce males, (I guess males since males cannot carry malaria. As Humans have twice as many female ancestors as male ancestors a modification to cause all females would be a much better genetic attack than males.). https://www.nature.com/articles/srep31139
Mosquitoes have been genetically modified so they only give birth to males in a new technique that scientists hope could help to wipe out malaria. The disease is spread by female mosquitoes, which pass on the disease when they bite humans. But researchers from Imperial College London have now tested a new method that introduces genetically modified mosquitoes to normal Anopheles gambiae mosquito populations, the main carrier of the malaria parasite. The GM insects had been altered so that they would not produce female offspring. In the first laboratory tests where the modified mosquitoes were mixed with normal insects, 95% of the eggs laid hatched into males.
This could (in theory) be down to some behavioural practices passed down, or some genetic abnormalities in ovulation, tubes, etc.
In practice these effects aren't hugely significant.
"You are more likely to have a boy if you have sex only once during your fertile period and a girl if you have sex more often. True This is based on the fact that female sperm live longer and swim slower than male sperm. If you have sex a few days before you ovulate the longer living and slower female sperm will be waiting for the egg. If you have sex just before ovulation the faster pushier male sperm will win the race. Whatever you do remember that really there is a 50:50 chance of having either a boy or a girl. Focusing on having a healthy baby is more important than its gender."
Many statistically very improbable situations do occur. Perhaps by chance a woman, and her daughters, granddaughters, great granddaughters, great great granddaughters, etc. only give birth to females.
Especially if the women give birth to just a few children each, it will be easy for people to keep track of all the children and notice that they are all female.
I once read there was a feudal fief that was inherited by heiresses for about six generations in a row because of lack of surviving male heirs.
If you don't want to come up with an explanation for the lack of male children, you could make it happen for just a few generations by the time of your story, long enough for people to notice and speculate that it is caused by a curse.
And if your story is a fantasy, perhaps probability changing magic is used in this world and a spell was used to make the probability of having male children zero.
There is in fact known to be genetic factors that predispose a person to have one or the other. This is part of the species ability to adapt to new conditions, via natural selection.
If it were just a simple non-genetic thing, with nothing but pure chance involved, then you'd expect birth gender ratios to be 1:1. But that's not what we see at all.
Worldwide, the average gender at birth is about 1.09 boys to girls*, with the gender differential narrowing as the population ages. What's going on here is that humans are tuned to produce slightly more males to make up for their slightly increased death rate.
In countries with long-term skewed gender imbalances (eg: China and India, with strong cultural preferences for male children, and Russia where 3 major wars on their territory in 40 years for a time left the ratio among adults nearly 2:1 female), its been observed that the birth ratios slowly adjusted to compensate. If there's going a shortage of the less numerous gender when it comes time to ..er..produce offspring, that makes the genes of parents who tend to produce more of that gender slightly more likely to get passed on to the next generation than the genes of other parents.
That of course in turn implies that there are indeed people who are genetically more likely to have one of the genders than the general population is.
So all you have to do is make your family one of those.
* - To be fair, this is probably thrown off by the world's two most populous countries having artificially skewed birth rates. The normal ratio in other countries throwing those two out seems to be more like 1.05. Interestingly, the only country in the world that has more women born than men is a small island nation with a traditionally matrilineal society (so their rate is likely artificial as well).
I am a fan of Parthenogenesis. Parthenogenesis (from the Greek for "virgin birth") is the development of an unfertilised female egg into a viable individual.
It is known to occur naturally in some invertebrate species, and is thought to have occurred in some birds and fish. However, there have been no natural cases where a mammal has given birth from an unfertilised egg, and it is likely such an embryo would be unviable.
The origin of the orgasm is to trigger the release of an embryo from the ovaries. We don't have that issue - humans have monthly cycles where an egg is released and disposed of.
As a basis for your scenario, a mutation could have occurred wherein the women's ovaries do not release an egg, except during orgasm (undoing several hundred generations of evolution), which starts developing on its way to the womb. By the time the spermatozoa reach the egg (assuming a male is present) the embryo will already have polarised, repelling any sperm.
Each woman's daughter is her own "clone".
In this little scenario, it would be possible for a girl to get pregnant when she starts experimenting with masturbating. Her parents won't believe this is a virgin birth or whatever and some local will be arrested for paedophilia.