The male. He carries an X and Y chromosome. If he gives an X, the fetus is female. If he gives a Y, the fetus is male.

The female fetus has a Y chromosome while the fetus only has two x chromosome.

A mother has no role in deciding the gender of an unborn fetus. Mothers always provide an X chromosome; fathers provide an X or a Y. If the fetus has two X chromosomes (one from mom, one from dad), then you'll have a girl. An X and a Y--it's a boy.

In humans, the male determines the sex of the fetus. The mother has 2 X chromosomes and each egg she produces just has an X. Males have both X and Y so they can have a sperm with X and a sperm with Y. A female fetus has XX, one from mom and one from dad. A male fetus has one X from mom and a Y from dad.

The X Files, though the fetus is nominally human. Scully is the woman.