Hormones Take Over

The gene only starts the process. After SRY fires, hormones do the building. Levels of testosterone and estrogen during specific prenatal windows shape the gonads, the reproductive plumbing, body composition, and the developing brain. Those hormone levels vary from one pregnancy to the next, which is why real bodies show far more spread than a textbook diagram of two tidy outcomes.

Here's the proof that genes alone don't decide the outcome: androgen insensitivity syndrome. These individuals have XY chromosomes and working testes that pump out testosterone — but their cells carry a faulty receptor and simply can't hear the signal. So development follows the default path. They are genetically male yet grow up with typically female bodies, and many never know until puberty.