Why does a human male determine the sex of the child?


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Because the male carries one X and one Y chromosome, whereas a female carries two X chromosomes. Therefore, all female gametes (eggs) will contain an X chromosome, whereas male gametes (sperm) will contain either the X or the Y chromosome.

When one sperm fuses with one egg, it will either be XY (male) or XX (female), depending on which of the male's sperm, the one with the X or the one with the Y, made it there first.