Premise: Assume that one man has a genetic mutation such that his gametes (sperm) never have an X-chromosome. Unlike most men (who have a roughly 50/50 chance to sire a male or a female child) this man can only sire sons.

Assume further that this mutation is 100% hereditary such that every son born to his line will have the same mutation, siring sons.

  • How long might it take for officials to notice that something was wrong?

  • Under what circumstances would this be declared a problem by the government, such that it might act to stop further procreation?

  • What cultural or sociological conditions would increase or decrease the likelihood of detection?

  • Is there any plausible scenario in which the result would be catastrophic population collapse in a region or worse?

  • 1
    $\begingroup$ For this this mutation will be inherited in the male-upon-male line it must affect a gene on the Y chromosome, but the tiny Y chromosome does little but flag the embrio as genetically male. Leaving aside the observation that the legal father and the biological father are not necessariIy the same, I see no way for such a condition to cause a population collapse unless it affects Genghis Khan; after all, men are just extra baggage when it comes to demographics; a mutation which affects only men is demographically mostly irrelevant. But I would accept it in a book, it's not unbelievable. $\endgroup$
    – AlexP
    Oct 6, 2017 at 20:37
  • 2
    $\begingroup$ @AlexP - this mutation will affect women as well - they will stop being born. $\endgroup$
    – Alexander
    Oct 6, 2017 at 20:51
  • 1
    $\begingroup$ such a mutation already exists in mice, the mutation is named funnily enough R2d2, ncbi.nlm.nih.gov/pmc/articles/PMC4868115 $\endgroup$
    – John
    Oct 6, 2017 at 21:48
  • 1
    $\begingroup$ @Alexander: How does it affect women? John has only sons, who have only sons, who have only sons. But all the other men are normal; they have both sons and daughters. What you will have is an imbalance between the number of men and women, but since men don't really count demographically this is a wash; you have simply reduced the effective size of the population to a certain degree. In order for the mutation to spread it would have to be advantageous. (And in humans it would need to be very advantageous, because human population is small genetically.) $\endgroup$
    – AlexP
    Oct 6, 2017 at 22:21
  • 1
    $\begingroup$ @AlexP - try to estimate the degree of population reduction. With every generation, Y-offspring will be only spreading, there is no natural mechanism for X-offspring to keep proliferating. Eventually, population should go to 0. $\endgroup$
    – Alexander
    Oct 6, 2017 at 22:28

8 Answers 8


How long might it take for officials to notice that something was wrong?

Several years. They would notice that the female to male ratio is going down and would wonder why, but for this to be noticeable it would need to affect at least 2-3% of population. Someone would start suspecting, and looking for, female infanticide. When this does not pan out, they'd start looking for some causes, investigating whether the phenomenon is local or not; there are chemical pollutants that mimic hormone activity and could be the cause of this.

But very soon someone would run a sperm check, the same kind of test that's done to increase the likelihood of male offspring. Or someone with the mutation might ask for female offspring - selecting gametes with X chromosome. And the technicians would scratch their heads and say "Sorry sir, you haven't any" and rush to publish their finding.

Under what circumstances would this be declared a problem by the government, such that it might act to stop further procreation?

Probably not for a long time. The government would declare it a problem, and surely screenings would be made available for those who wanted, but what would be the point of stopping procreation? Male children have traditionally been preferred in many countries, so this might not even be seen as a problem by many, and could be regarded as a blessing for some.

On the other hand, from some simplistic simulations I've run, without a working test and some degree of enforcing, either by social pressure or governmental fiat, the population appears to be doomed.

What cultural or sociological conditions would increase or decrease the likelihood of detection?

Possibly if the country already had a male/female imbalance due to a policy such as "only one child for family", combined with a traditional preference for male offspring that had resulted in the disappearing of female fetuses (or even newborns). Then, a mutation that has the same effect would be hidden for longer (one more generation - twenty years? Twenty-five?). But even there, there would be someone wanting females - for example, to join two families - and the truth would emerge.

For the same reason, any sort of detailed sperm check for whatever reason would reveal that some guy has no X spermatozoa. Further tests would immediately follow. So any advanced society where genetic screenings are performed (to, say, reduce the risk of conceiving children with genetic syndromes) would see the game discovered in a matter of days once an affected individual entered in the tested pool.

Mandatory genetic testing against genetic diseases for everyone would trigger discovery as soon as the first affected individual decided to have children - say some twenty to forty years after the mutation took place at his conception.

Is there any plausible scenario in which the result would be catastrophic population collapse in a region or worse?

Yes. At first, in absence of tests, the mutation will spread more or less linearly at each generation (assuming the generation size remains constant), and all scenarios lead to extinction:

48.10% F, 51.60% M, 0.30% X   // Linear growth
48.37% F, 50.87% M, 0.76% X
47.78% F, 50.50% M, 1.72% X
48.28% F, 48.80% M, 2.92% X
45.81% F, 48.76% M, 5.43% X
42.63% F, 48.39% M, 8.98% X
42.25% F, 41.55% M, 16.20% X
36.71% F, 35.84% M, 27.45% X  // Curve starts to flex
28.99% F, 27.42% M, 43.59% X
19.90% F, 20.39% M, 59.71% X
13.83% F, 13.33% M, 72.84% X
7.84% F, 9.13% M, 83.04% X
4.38% F, 4.97% M, 90.65% X
3.09% F, 2.13% M, 94.78% X
1.76% F, 1.27% M, 96.97% X
0.30% F, 0.30% M, 99.40% X
0.20% F, 0.20% M, 99.60% X
0.00% F, 0.00% M, 100.00% X

But if we introduce a testing when females are 10% of the population (pretty late if you ask me), which decreases the chances of a fertile "YY" mating to 10% of normal (this takes into account testing errors and people marrying knowing the consequences and having children nonetheless):

14.86% F, 14.78% M, 70.35% X
6.91% F, 8.33% M, 84.75% X    : X < 10%, introducing tests
25.64% F, 23.78% M, 50.59% X     // Ratio immediately drops
40.08% F, 41.41% M, 18.51% X
49.37% F, 45.63% M, 5.00% X
50.80% F, 48.31% M, 0.88% X      // Decrease becomes 1:10
52.61% F, 47.31% M, 0.08% X
48.20% F, 51.80% M, 0.00% X
Mutation dies out

Other scenarios see a maximum of two children per couple, and since a viable couple needs one female, the population declines rapidly:

49.80% F, 50.10% M, 0.10% X, population 100%
48.80% F, 50.80% M, 0.40% X, population 99%
48.87% F, 50.31% M, 0.82% X, population 97%
52.00% F, 46.32% M, 1.68% X, population 95%
48.68% F, 47.17% M, 4.15% X, population 98%
49.38% F, 44.28% M, 6.34% X, population 96%
40.53% F, 44.63% M, 14.84% X, population 95%
37.27% F, 38.44% M, 24.29% X, population 77%
32.23% F, 28.05% M, 39.72% X, population 57%
24.32% F, 23.24% M, 52.43% X, population 37%
Introducing tests
35.56% F, 40.56% M, 23.89% X, population 18%
49.22% F, 46.88% M, 3.91% X, population 12%
54.76% F, 45.24% M, 0.00% X, population 12%
Mutation dies out

If, in addition to tests, a third child is encouraged:

Introducing tests, maxc=3
42.69% F, 36.26% M, 21.05% X, population 17%
49.77% F, 44.75% M, 5.48% X, population 21%
49.85% F, 48.62% M, 1.53% X, population 32%
54.19% F, 45.81% M, 0.00% X, population 48%
Mutation dies out

With a testing of 50% efficacy and a policy of allowing a third child only when population is below threshold, 2 otherwise, the population stabilizes around a 6% of mutations, oscillating between 90% and 130% of threshold.

Of course, real world conditions - people ignoring the tests and/or shirking the children limitations and/or not having all the children they can - may shift these results considerably.

  • 2
    $\begingroup$ Surely it would take more than a few years. If it started with a single individual, it was take several generations before the population gender balance would change measurably. $\endgroup$
    – smatterer
    Oct 8, 2017 at 1:51
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    $\begingroup$ If there were THOUSANDS of males with the same genetic defect, a few years. But if it started with only ONE male, then it may never be noticed. Unless that one male was very, very prolific, and impregnated many, many females. There are many families that have long runs of male-only babies in them, through multiple generations. Remember the all-male Cartwrights? But remember, for one male producing only male babies, there are 3 billion producing normally-distributed sex ratios. $\endgroup$ Oct 8, 2017 at 3:22
  • $\begingroup$ @smatterer, JustinThyme - you are absolutely correct, of course. Edited answer. I was thinking of a sizeable seed population, and my simulations started with 1% of affected individuals. But of course it would start with one, and it would take several generations to reach 1%; and each generation would take a good twenty years at least (more, in countries where people tend to have children later in life). $\endgroup$
    – LSerni
    Oct 8, 2017 at 9:58

If not identified and taken under control this mutation will result in the extinction of the species, especially if it is linked to the Y-chromosome. That is if sexual reproduction is the only option. Genetic engineering and cloning will help to avoid the extinction.

In your suggested scenario, the number of women will start to decline. At first, it will be happening slowly, but as the gene spreads, fewer couples will be having daughters. This will lead to huge social changes (for example, increase in a number of marriages and their stability) and possible collapse of the societies (not immediately, of course) since skewed sex ratios favouring males are linked to social unrest.

The reaction of governments and societies and when the mutation is noticed will depend on the level of medical science and/or genealogy traditions. Sufficiently advanced societies with the understanding of genetics and developed traditions of preserving and studying family histories would have an advantage. I believe that they will try to find all men with this mutation and sterilise them in order to avoid extinction.

I would also guess that a society that places a high value on sons will make some associations with these men unique ability to produce sons only. They may even see it as an advantage. Although, even those societies would probably eventually start to stigmatise those men.

For the spread of the gene, you need to decide on your setting, fertility rates, marriage patterns, and so on. Without these, any calculations and assumptions can easily go wrong.

  • $\begingroup$ I'm not entirely convinced that it's more males means more unrest, but it could merely be that being out of a particular male:female ratio leads to unrest. Mexico, many countries in S. America, and many African countries have more women than men, and they do have their problems. (Based off of a survey of countries by sex ratio: en.wikipedia.org/wiki/List_of_countries_by_sex_ratio ) $\endgroup$
    – PipperChip
    Oct 6, 2017 at 22:40
  • $\begingroup$ @PipperChip, there is research linking skewed sex ratios to increased violence and social problems. It is not conclusive, though. Prevalence of women, indeed, leads to different problems, for example, to a possible increase in gender inequality. However, research shows that women tend to cooperate and invest back into family and community more than men. Take a look at microcredits for some interesting data in this regard. $\endgroup$
    – Olga
    Oct 7, 2017 at 0:05
  • $\begingroup$ @PipperChip Look to China for your answer. China has some 30 million more males than females, due to cultural mores and the one-child policy. Social upheaval was so dramatic that they were forced into a two-child policy. $\endgroup$ Oct 8, 2017 at 14:26

There are more than 3 billion men in the world, if a Y+ man has an average of 6 Y+ children (say it has some psychological effects too) it would take 12 generations to get there. Until the last generation the majority of men don't have the gene, so there is more than a hundred (probably more like 300) years to realize we have a problem before solving the source is not good enough. Even once we reach the gender skew in more skewed parts of the world we still have another generation before we would expect normal males (and the species) to be vulnerable to extinction.

But it wouldn't spread that far that fast from a village. In its home village it might cause problems, as many sons might be expected over a few generations to marry into most of the local families. This would then produce a local surplus of males and a dearth of females. At this point scientists might notice and would be interested at the departure from the expected sex ratio at birth.

The village could probably import brides from neighboring places, but as it exports males other nearby communities would also be effected, soon making it clear what is happening. While some cultures might value this on principal marrying foreigners isn't all that common in much of the world, and pretty quickly the drawbacks would become apparent in the original district.

If it gets to a city first things might be a little more interesting. If siblings don't stay close there would be little reason to lump them as a demographic so science would not be interested, anecdotal stories of ten grandsons and no granddaughters are just curiosities. By the time anyone found a larger demographic like a city or region was a couple sigma out of normal the gene would be pretty wide spread, but there would not be immediately clear evidence of the cause. And if you are before computers the tracking required might make finding the cause at all doubtful.

In this case village girls get attracted to cites as the fewer city girls make finding a well off guy virtually guaranteed. But villages won't be able to support the ratio and populations will then decline. As the population in cites declines it gets easier for the villages not yet exposed to the gene to replace missing females in cities, but more villages would have accepted outside males at some point leading to my first course.

Even the densest scientists would see what was happening by the time cities populations decreased, but that is one generation after interference can keep population stable, and more decline is expected before recovery no matter what. Again it would become clear that the gene isn't good and it would be avoided, but this time much of the connected civilization would have been exposed and recovery would need to come from unconnected pockets.

Once the gene is identified it won't really matter long term.

Imagine two groups of people, the Xs and the Ys, the Ys have no females and therefore their population isn't meaningful to the size of the next generation. The Xs need to produce one female for every female in their group, plus one for every female that joins the Ys. This thinking changes the nature of the problem form a gene spreading one to a simple surplus male problem, even if it is renewed from generation to generation. If we end up with a stable population of Xs it doesn't mater how many Ys we have.

The Fisher Principle says evolution would be expected to eventually solve it. But humans aren't that pressed, and we could pretty easily cope with a skewed sex ratio, you just might have first world birth rates more like third world ones to keep a stable population with extra males about.


An Issue With the Premise

You never stated if the gene is passed on to the next generation. This is super important. If it is never passed on, this mutation is odd, but not an issue for society at large.


$P(n_{boys}) = (\frac{106}{200})^{n_{boys}}$ is the equation that gives the rough chances of having a family of only boys with $n$ children. For $n=5$, the odds of this happening become a little above 4%! (The odds of two boys in a row- about 28%.)

Adding the odds of children to the mix, and even with two children to each couple, the odds of this happening are a little under 2% after the first 4 grandchildren. For 3 children each, it's 0.05% after the last grandchild. I would expect people to wonder about if after the 1st generation has children. (The initial man/wife pair is generation 0.)

This is also assuming that these people live in a society where knowledge of grandparents and cousins is common, and no one is put up for adoption, becomes sterile, dies before their time, and so on.

If genetic screening for this gene (or all genes) is available, this theoretically can be caught with the initial man, and potentially contained with his children. Of course, just because we can sequence a genome doesn't mean that we instantly know what those genes do.


We, as a species, seem to be pretty terrible about coming together and doing something for the good of the species. This is especially true if it involves individual sacrifice. Pick a topic, and see how bad humans are at having a single, unified stance: capitalism, climate change, toilet roll facing, Kirk vs Picard, etc...

In short, unless the people who want the human species to continue are in charge and can implement a eugenics program, this problem will spread, possibly dooming humans.

  • $\begingroup$ Pick a topic: medical emergency. Reference: SARS and HIV. We're better at dealing with medical problems than almost anything else. You're right about querying the premise. It's unlikely the mutant gene will spread far without being detected. Then genetic screening can filter out the harm. Female offspring will be at a premium. There are far worse genetic health problems. This is so unlikely to doom humans. Plus one. $\endgroup$
    – a4android
    Oct 7, 2017 at 8:10
  • $\begingroup$ @a4android and none of those require people to become sterile. That's the key here: personal sacrifice for the good of mankind is not mankind's strong suit. $\endgroup$
    – PipperChip
    Oct 7, 2017 at 16:34
  • $\begingroup$ But there ARE families with five boys. In fact, in a family of four boys, the odds of the next being a female are substantially less than 50/50. Some even propose that there is some form of immunity function happening - that having a baby of one sex sets up an immunity to having a baby of the opposite sex the next time, and after four of just one sex, this immunity is very strong. $\endgroup$ Oct 8, 2017 at 2:59
  • $\begingroup$ @JustinThyme If you roll enough dice, you get every result. Any statistical model will need to make assumptions, and there is always the off chance that it's a fluke. I suppose the math I did in this post is an argument for such immunity functions. shrug $\endgroup$
    – PipperChip
    Oct 8, 2017 at 19:03

Most importantly to any consideration, it must be noted that the Female sex is the default sex. That is, unless something specific happens in utero, a female will be produced, irrespective of the presence or absence of the Y chromosome.

The Y chromosome is NOT the only thing that determines the sex of the offspring. There are mammals without the Y chromosome, and they still have males. The human Y chromosome has atrophied to such an extent that it is posited it will eventually disappear. In the same vein, female sex determination is not completely dependent on the absence of the Y chromosome. Is the Y chromosome all that is required for sex determination?

But there are basically only three ways that a male could produce only Y chromosome gametes.

First method, all of the cells in his body would have to be YY. If so, he would be horribly maladjusted, as the genes on the X chromosome are vital for human survival. Mate selection would be troublesome.

Second method, X and Y gametes are produced normally, and something causes the X gametes to be destroyed (like an immunological reaction). Immunology is not on the Y chromosome, so it would have to be on some other chromosome, which means it would not be 100% passed on. Sperm count, and thus fertility, would be cut in half.

Third method, that X and Y gametes are produced normally, but the X sperm are severely disadvantaged such that the X sperm are unlikely to cause fertilization. Sex selection is already built in to the male sperm, since male Y sperm swim faster but die quicker. This has altered the ratio of male to female births, but has not severely altered it. To be 100% effective, the male sperm would have to me completely nonmotile. Again, effective sperm count and thus fertility would be substantially reduced.

It is hard to envision, under any of these methods, that this mutation would become established in the human population, without substantial help from in vitro fertilization, in which case the process is moot. No need for such a mutation, X sperm would just be screened out, as is done currently.

If you want hard statistics on the societal repercussions of a male birth preference, investigate the research from China. The one-child policy has lead to there being over 30 million more males than females, in just three or four generations, and this was due not to just one male, but to widespread selective abortion and infanticide. The societal implications are substantial and were unforeseen. Unbalanced sex ratios tend to be self-balancing in the end. Males that produce female offspring reliably, in the family background, would become preferred mates.

  • $\begingroup$ You do not mention that 46,XX males and 46,XY females are infertile. 46,YY males do not exist at all. However, 47, XYY males are very manly and fertile. With some handwaving they could produce sufficient amounts of Y-only sperm :) $\endgroup$
    – Olga
    Oct 8, 2017 at 10:14
  • $\begingroup$ @Olga It's that 'handwaving' bit. With 'handwaving', anything is possible.The trick is, to make that XYY genetic defect a result of a gene on both the Y chromosomes, so that it is 100% hereditary, and to totally suppress the X chromosome. otherwise, the required 100% is not achieved.. $\endgroup$ Oct 8, 2017 at 14:11

The first generation - that is, the first person with a YY chromosome.

The Y chromosome is exceedingly small, it has lost 90% of its viable genetic material. See for instance this

The Y chromosome has very little genetic material left, and any viable offspring needs the X chromosome genes to survive. A YY baby would have little chance of survival.

In fact, eventually there will be no Y chromosome in the human species.

The good news is, it is not needed to produce a male. It is only a very minor factor in sex determination. Everything else is done by hormones and such.

  • $\begingroup$ the babies will get X chromosome from their mothers... An egg with an X chromosome + sperm with a Y-chromosome = XY baby (male) $\endgroup$
    – Olga
    Oct 7, 2017 at 3:46
  • $\begingroup$ The issue is the YY male that started this process. No X chromosome. $\endgroup$ Oct 7, 2017 at 14:02
  • $\begingroup$ I think you are confusing sperm and somatic cells. You also disregard the way sexual reproduction works. Sperm has only half of the chromosomes... $\endgroup$
    – Olga
    Oct 7, 2017 at 19:18
  • $\begingroup$ @Olga Sperm has only half of the chromosomes, but the original split came from a complete chromosome. That complete chromosome came from a cell from the host body. So either that original cell was YY, as would every other cell in the body, or the process only half-worked, and the X part of the division was somehow destroyed. That is, there can be 'all Y sperm', but there can never be a YY originator cell unless the entire body was YY. $\endgroup$ Oct 8, 2017 at 3:04
  • $\begingroup$ first of all, nowhere the OP states that the original mutant is YY male. But it does not matter at all. It takes two -- a man and a woman -- to make a baby. Offspring of a man with Y-only sperm will receive X chromosome from their mother(s). All viable children will be XY males that can produce only Y sperm... Please note, the OP does not ask for an explanation of the mechanisms of this mutation. I assume it is handwaved. $\endgroup$
    – Olga
    Oct 8, 2017 at 9:39
  • How long would it take to be noticed?

    That depends where it started. If it started in a society where most families have many children (typically poor or per-industrial societies), it may take a couple of generations to be noticeable. If I have six children - all boys - and my parents and five brothers all have many boys and no girls, it becomes something that is worth medical investigation. Even without modern medicine, after a few generations it would become known that there were some families that only had boys.

    On the other hand, in societies where most families have only one or two children, it is not so obvious. If I have two sons and no daughters and so do my parents and brother, it is not a coincidence worth investigating. It is more likely to be discovered when a couple who want a daughter used some type of assisted reproduction technology to try to select their childs sex and discover that there are no X-baring sperm.

  • The effects on Society

    If nothing was done about it there would be a gender imbalance in the population. When there are excess men with no prospect of marriage and family, it is likely that there would be institutions like the military or monasteries which provide a substitute for the family environment for single men. It is also possible that women would give more priority to child-baring as that "skill" becomes rarer and hence more valuable.

    I suspect that the imbalance would be self-correcting or at least self-limiting. If there are many more men than women then women can be more fussy in their choice of partner. Even without genetic testing technology, it will be known which men are from affected families. All else being equal, women will prefer husbands from unaffected families both because they want to have daughters and because they want their sons to be unaffected.


The man without an X chromosome would likely also be sterile.

  • 1
    $\begingroup$ He will get one from his mother in the majority of cases... The mutation affects sperm production, not the genetic composition of males. $\endgroup$
    – Olga
    Oct 6, 2017 at 21:39
  • $\begingroup$ Welcome to Worldbuilding, @DavidPerreaux, sorry about the initial negative reception to your answer. Your answer can be improved by explaining sterility and any other effects occurs. Also, how this mutation could produce a medical or population catastrophe. Have fun on Worldbuilding! $\endgroup$
    – a4android
    Oct 7, 2017 at 0:35
  • $\begingroup$ I would posit that the male would also have a host of other genetic problems. and probably have a short life. The problem is in the wording of the question - there can be 'all male sperm, carrying only a single Y chromosome, and never an X chromosome', but there can not be a YY cell unless the host is ALL YY chromosomes. Cell division does not allow for an X chromosome in an XY pair to suddenly turn into a Y chromosome to create a YY cell. $\endgroup$ Oct 8, 2017 at 3:11
  • $\begingroup$ And if only HALF of the gametes survived (just the Y gametes) then his fertility would be cut in half. I strongly suspect he would have a very difficult time having children, with a sperm count of only 50%. $\endgroup$ Oct 8, 2017 at 14:37

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