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My son studies genetics at school right now and they have been covering gender. It's well known that slightly more boys are born than girls. However, originally there's an equal number of sperm cells with X and Y chromosomes. From what I've read it sounds that fertilization is affected by different factors but it seems that on average it's about equally likely? At school, they told them that more fertilized female "embryos" die than male?? Could it be true? Question: Do you know how it happens that there is an equal number of X, Y-carrying sperm cells, but more boys are born?

Sleepy Hollow
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    This page has quite a nice overview of the topic: https://en.wikipedia.org/wiki/Human_sex_ratio#Factors_affecting_sex_ratio_in_humans but as far as I know, it's not well understood. – Jam Apr 12 '18 at 23:05
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    The hypothesis that male embryos are more viable seems reasonable, but I've not seen any data to back that up. Also, it's purely speculative but you could imagine how particular proteins expressed with X or Y chromosomes could change how an embryo develops, which in turn could change how viable the embryo is. – Jam Apr 12 '18 at 23:13
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    The Y chromosome is smaller than the X. So sperm carrying the Y chromosome are very slightly lighter, and faster. I'm not an expert on this, but I wonder if it might be a possible reason. – Karl Kjer Apr 12 '18 at 23:31
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    @Jam That link says that sex ratio decreases as the father gets older. Maybe older men have a higher proportion of defective Y chromosomes? That could cause a greater number of male embryos to fail and bring the male to female ratio down. – user137 Apr 13 '18 at 02:27
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    This post seems to adress the question https://biology.stackexchange.com/questions/27582/why-do-we-have-more-male-infants-born?rq=1 – Untitpoi Apr 13 '18 at 15:37

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