Technically, the father determines the sex of the offspring. There
are two sex chromosomes: the "X" and the "Y". If an embryo has
"XX", it will develop into a female, while the "XY" combination
results in a male offspring.
To understand why, a little knowledge of Miosis is needed (this is
the process by which sex cells are created (eggs and sperm)).
Miosis is related to Mitosis, in that the end result is the
creation of two cells from one. Miosis, however, ensures that half
the genetic material is handed to each "daughter" cell, while
Mitosis results in equal and even distribution (cock) of genetic
If all men are "XY" (they are), then the process is pretty
straightforward: Each of the sperm must have half the normal
genetic material. This means that each must have either an "X" or a
"Y" chromosome. If you're a dude, 50% of the sperm will result in
male offspring, and 50% will result in female. Females have two "X"
chromosomes, so even when only half is replicated, there's a 100%
chance an "X" is present. Everyone needs two sex chromosomes to
grow, so, therefore, if 100% of the time the mom is giving an "X",
and half the time the pop is shooting "X"'s and "Y"'s, it's fully
the dad's genes that decide of the offspring is male or female.