Question

Why does the male parent's genes determine the sex of the baby?

Answer 1

The mother has two 'X' chromosomes so all the mother has to offer to the new born are 'X' chromosomes the father has and 'X' chromosome and a 'Y' chromosome so a father either contributes an 'X' chromosome or a 'Y' chromosome. That is why the father determines the sex of a child.

Answer 2

Consider this:

Each parent has two sex chromosomes.
The mother, as a female, has chromosomes XX.
The father, a male, has XY.

The mother will pass on one sex chromosome to the baby, as will the father.
The mother can only pass on an X.
The father, however, has a 50/50 chance of passing on an X or Y.
XX would make a female and XY would make a male.

Simply put, the sex chromosome from your mother is fixed, it's going to be an X no matter what. It's which sex chromosome your father passes on that determines your sex.