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It's worth starting by noting, that male and female births are not 50-50. While conceptions are 50-50, births aren't and there are mechanisms that terminate pregnancies unsuccessfully that have different likelihoods based on gender.
While it makes sense that the value is near 50% for humans it's not exactly both in reality and in computer models I did for human evolution (and it surprised me).
Sexual selection is very useful. In humans, mitochondrial DNA is only passed maternally and we see that evolution reduced the number of mitochondrial genes to a minimum.
For each of the chromosomes we get one from our mother and one from our father. There's no easy way to give 1.25 from the mother and 0.75 from the father. If we get two of one of the chromosomes or none from one parent in most cases the pregnancy terminates unsuccessfully and in the few remaining cases, it produces severe harm (like down syndrome).