When a woman is pregnant, is the chance of having a girl or boy equal 50/50? Or are there like some biology stuff determining the odds?
In: Biology
Yes, it is biology stuff. A woman has XX sex chromosomes and a man has XY†. Egg cells and sperm cells are special because they only have half of the genes needed to make a person, they combine and now you can start down the path to a baby.
Since eggs always have X, how do sperm choose? Sperm are made in pairs, by splitting a cell. This gives one X sperm and one Y. At this point the odds are always 50/50. But there are many steps, as others have described, between production and fertilization.
† It’s actually a little more complex as there are unusual conditions with 3 sex chromosomes, but that’s a statistically unlikely edge case and beyond the depth of ELI5.
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