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It is known that some species are more likely to born male than female (and some maybe vice versa). I understand that it is due to evolutionary perspective (average number of adult males is close to that of females), I want to know about how it is regulated.

What I thought is that spermatozoa are made by half as Y and half as X chromosome. Then mitosis and meiosis and the probability that one of them will die is equal for both types. So, somewhere I feel lost.

Also, can it be regulated by female body or not? According to this paper claiming that hormone levels in women affect the likelyhood to born son. Possibly it is just because they choose males that are more likely to have more Y spermatozoa than X, but if it is some chemical mechanism to regulate that I'd like to know.

I'm not asking only about humans, mechanisms can be similar in almost all mammals. I'm not asking about other species: I know some are regulated by temperature, etc. This question is only about mammals.

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    $\begingroup$ I think you are only asking about mammals. Males and females are not universally defined by chromosomes, or genitalia. They are defined by the size of their gametes. Female gametes are larger than male gametes. Some fish switch from males to females as they grow larger. The sex of some reptiles are determines by temperature. So are you asking ONLY about mammals? $\endgroup$ – Karl Kjer Nov 22 '17 at 13:19
  • $\begingroup$ You need to specify an organism. The process differs widely throughout the animal kingdom. $\endgroup$ – David Nov 22 '17 at 21:59
  • $\begingroup$ @David I guess mechanisms are similar throughout all mammals. $\endgroup$ – rus9384 Nov 22 '17 at 23:17
  • $\begingroup$ Probably. But birds, fish and fruit flies... $\endgroup$ – David Nov 22 '17 at 23:19
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    $\begingroup$ Tags are for indexing. Anything central should be in the question. $\endgroup$ – David Nov 22 '17 at 23:30
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In theory spermatozoa are indeed produced in a 50:50 X:Y ratio. However, there are factors which can change the ratio, such as hormone-disrupting agents or even food.

Best known factor in the female body is timing. There are different theories of which The Shettles method is the most well known I think. In this theory they think that boy sperm are faster than girl sperm, but don't live longer. So to have sex around the date of ovulation highers the chance of getting a boy (their sperm is faster), but for a girl sex should be planned a few days before ovulation (they live longer). It is all theory, of course, so there are no guaranties.

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  • $\begingroup$ Interesting information. Official / credible sources would radically improve this answer, though. $\endgroup$ – rotaredom Nov 22 '17 at 14:27
  • $\begingroup$ I have changed the link for Shettles method to wiki, where there are more links to NEJM papers, the results are conflicting though between both NEJM studies, so credible sources are not always 100% waterproof. I speak of a theory therefore, this is stuff I had in high school Biology class when I was still a kid. $\endgroup$ – user35628 Nov 22 '17 at 14:45
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    $\begingroup$ If you had linked to the wiki references directly yourself, it would be easier to see that they are very old references, and that the most recent ones do not support the Shettles method. $\endgroup$ – swbarnes2 Nov 22 '17 at 22:10
  • $\begingroup$ @swbarnes2 Please link to more recent ones, to support your comment. $\endgroup$ – user35628 Nov 23 '17 at 8:55
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    $\begingroup$ Okay, I can link to what you indirectly referenced... ncbi.nlm.nih.gov/pubmed/11191075 ncbi.nlm.nih.gov/pmc/articles/PMC1440662 $\endgroup$ – swbarnes2 Nov 27 '17 at 18:08
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What processes regulate the sex of offspring?

The processes rulating the sex of offspring may occur in both males and females and be direct or indirect.

What I thought is that spermatozoa are made by half as Y and half as X chromosome. Then mitosis and meiosis and the probability that one of them will die is equal for both types. So, somewhere I feel lost.

This may not be entirely true. The phenomenon called "Meiotic drive" may change the ratio from 1:1. Meiotic Drive occurs through selfish genes. See "Selfish-gene theory" for more info.

Also, can it be regulated by female body or not?

if it is some chemical mechanism to regulate that I'd like to know.

Possibly, there is such in mammals. There is a bunch of mechanisms called "Cryptic female choice". Some of them are chemical. Although, the article is about birds, it can be the case similar mechanism is present in mammals.

Given that higher androgen level exposure in mothers affect make their offspring more male-typical in playing behavior we might consider that in general makes offsrpings more masculine. This would be a good reason for female organisms with higher androgen levels to favor sperm with Y chromosome present.

Possibly it is just because they choose males that are more likely to have more Y spermatozoa than X

Yes, this is possible. But considering cryptic female choice we may say the same for males. Some of them may seek those females who are more likely to get pregnant with particular sex spermatozoon.


Answering by my own since only now I occasionally came across with "Cryptic female choice" article.

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Both can play a factor across mammalia.In humans it is difficult to study effectively (we can't lock people and cages and mate them in controlled patterns, nor will most parents volunteer to hormone injections for their fetus) So we have less data on humans than for other species.

In other mammals we have had more luck and a variety of mechanisms exist, often several competing mechanisms withing the same species even. Not surprising if you think about out what gender the males genes "wants" and what ratio the females genes want will not always coincide. In rats for instance the R2D2 gene (I shit you not) prevents female offspring by preferentially differentiating germ cells so only those that carry it become sperm. The gene sits on the Y chromosome so it is selfishly passing itself on at the cost of often wiping out populations becasue they have no females. this is the most extreme mechanism know but it illustrates the sheer variety that exist.

dominance and stress are often good predictive factors, while known mechanisms include Hormones driven differential implantation, differential ova production, differential spermatozoa formation, and differential sperm survival. More mechanisms almost certainly exist but the search for them is relatively new so it is not surprising most studies will list them as potential influences.

Many mechanisms actually makes sense evolutionarily speaking, since there are several competing factors preferring one sex or the other as far as the parents genes are concerned. For instance if the ratios get too far away from the normal ratios it creates a n increasingly strong advantage to produce offspring of the sex in the minority. Another factor is parenting style of the species, in more or less monogamous species the sex has little impact while if you are a species that has harems females are sure thing with low pay off (numbers of offspring) while males are high risk high reward, low chance of reproduce but those that do have LOTS of offspring. Even things like for genes on the Y chromosome every female is a failure is a factor. You mist also consider that chromosome profile is not the only controlling factor, sex phenotypes that do not match their genotype is surprisingly common in mammalian.

Source

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