Which chromosome determines the sex of a baby?

The sperm from the father can supply either an X-bearing or a Y-bearing chromosome. The egg from the mother can only supply an X-bearing chromosome. An XY combination produces a boy baby. An XX combination produces a girl baby. It might seem that the father therefore determines the sex of the baby, but some recent evidence suggests that the mother can select either an X-bearing or a Y-bearing sperm. See the Wikipedia article on "Maternal influence on sex determination."