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 material.
If all men are "XY" (they are), then the process is pretty straight forward: 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 or for other matters, people like Tim Astembury