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Are there genetic factors that biase the sex ratio of offsprings for each person but average to 1:1 for the entire human population, or does the 1:1 ratio apply to every single fertile person?

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    $\begingroup$ The ratio is not 1:1: biology.stackexchange.com/questions/27582/… $\endgroup$ – kmm Sep 3 at 23:30
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    $\begingroup$ @kmm The question still stands -- is the ratio a statistical average or applicable to individuals? $\endgroup$ – arax Sep 4 at 6:50
  • $\begingroup$ According to this interesting article (in a reputable journal) two factors that are apparently important are the age and marital status of the mother: women over 35 are (slightly) more likely to have girls, and single mothers are (slightly) more likely to have boys (but the overall ratio of course is close to one (boys/girls=1.06 or thereabouts)). Published in 1996, maybe there are follow-up studies? $\endgroup$ – user1136 Sep 4 at 14:35
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The ratio you are referring to is indeed computed aggregating multiple observations and thus it is a "statistical average".

Any individual (or couple, in this case) can have a specific ratio that differs from the average one. Many factors may affect sex ratio, among which sperm/egg viability, chromosomal aberrations, and hormones misbalance are the ones usually affecting the fertility of a couple and they can also bias the sex ratio.

Many other factors can play a role. Different populations have slightly different sex-ratio and even smoking can skew the sex ratio!

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  • $\begingroup$ The question is not very well asked but I suppose the OP does not ask whether any given couple can have more offspring of one sex than of the other but rather whether there is genetic variance in the sex-ratio. $\endgroup$ – Remi.b Sep 4 at 15:41
  • $\begingroup$ @Remi.b I have interpreted the question differently. I will wait for the OP to comment to this answer or to clarify the question. If needed, I will adjust my answer. $\endgroup$ – alec_djinn Sep 5 at 6:31

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