The father. The mother gives an X chromosome (she has XX) and the father gives either an X, which makes a girl, or a Y, which makes a boy (he has XY).
And Patryk is wrong anyway. ONLY the father determines the gender of the baby. Mom can only give an X, but both males and females have one X, which is given by Mom. Dad can either give the X which makes it XX for a girl, or a Y which makes it XY for a boy.