@gizmorazaar Yeah according to the study it’s a significant but not a huge effect size.
Just noting that it’s always dependent on the male, so your brothers having daughters is relevant, but for the case of you and your brother’s daughters’ kids the sex ratios are irrelevant to your own family line- they’re influenced by the male partners’.
@crocodilehuricane Anecdotal, but this holds true for my wife’s family. She is one of seven - oldest is a boy followed by six girls. She has a cousin that has had seven children too - oldest is a boy, followed by six girls
@elam84 In humans there is a slight prenatal bias toward males at about 51.3% (source below). So, with some basic probability. The chance of two boys is 0.513 * 0.513 * 100 = 26.32%. The chance of two girls is 0.487 * 0.487 * 100 = 23.72%. And the chance of mixed sibs is 0.487 * 0.513 * 2 * 100 = 49.97% since we could have either B/G or G/B.
@grahammartinroyle i think the question was more whether this probability is independent of the sex of the couple’s other children. (to make the distinction: the figures you’ve given would still be true if, all other things being equal, 51.3% of couples had only boys and 48.7% had only girls)
I believe there's also some studies where certain things are genetic. So for example, if the family seems to concieve boys across generations, then there's more chance for you to conceive boys and vice versa.
That seems to be the case for my husband's family. His whole generation are boys and so far, our children's generation, we've all conceived boys. Across 3 grandsons, we have produced 5 great-grandsons.
Having said that, his mother's generation was 2 girls and a boy so......who really knows?
@aldredian We discussed this a bit in one of my genetics courses. A stressed mother is more likely to conceive XX offspring, while the normal species distribution tends to be 51/49 in favor of XY offspring. We're not sure whether this translates into an increased rate of miscarriage in XY embryos or if egg cells begin to show greater preference for X chromosome sperm, though.
Under normal meiosis, we would expect to have an approximately 50/50 chance for each biological sex, so it really is luck of the draw for most people.
@jcthesword Just spitballing, could a stressed mother create a more hostile environment which favors XX offspring? I read somewhere that X sperm can survive more hostile environments than Y sperm.
@cookiedion79 That's a possibility. One of my personal pet hypotheses is that because the X chromosome is hardier to stress than the Y chromosome, sperm in general are much less tolerant of stress than the egg, and we can assume that would-be fathers are undergoing the same environmental stressors as the mother, it might be that the stress is causing early denaturing of Y chromosome sperm. This would decrease their concentration in an ejaculate sample which in turn increases the likelihood of fertilization from X chromosome sperm.
@elam84 Not with the kind of sample size you’d have in one family. But if you have a pattern on the male side, like my husband’s family is 7:1 boys over 3 generations and 50-ish children, you might reasonably hypothesize that there is some particular aspect of that paternal line that favors gametes with the Y chromosome. But what that is and how to know whether it’s just a weird statistical quirk for any given family, I couldn’t tell you. I do know that I would not be shocked if we only ever had boys no matter how many children we had (though I’m definitely not planning on having enough children to have a statistically significant result).
@jluponeage I'm thinking there is a genetic factor as well. My friend comes from a very large family and her generation (her and her cousins) started having kids a few years ago. The current score is 26 boys vs 3 girls. The statistical chance of that is pretty damn low.
@drita Yes, but with billions of people on earth it’s going to happen to some families. It’s statistically unlikely to win the lottery, but people still do.
@drita It would probably only make sense to count the “male” line though (kids of male cousins who are sons of brothers), at least for this x/y chromosome ratio in the sperm question.
Separately I have read that some women have more acidic or basic vaginas that are more conducive to the survival of either male or female sperm. Which would theoretically be inherited mother to daughter…
