it's quite simple actually.
see, because males have an X and a Y chromosome, during early embryological development, the Y chromosome will cause something in the gonads to secrete a hormone that inhibits Mullerian ducts which then later, dies off. if not for this inhibiting hormone, the Mullerian ducts will develop into ovaries. on top of that, the secretion of testosterone will lead to further development of the male sexual organs.
vice versa, in the female, because they have two X chromosomes and no Y, they won't produce the Mullerian-inhibiting-substance and because there is no testosterone, their gonads develop into ovaries and not into male sex organs.
then, further development of gonads goes off from there!
--Joanne :P
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