Biologically/Genetically speaking, what determines whether you have a boy or a girl?

The other girls answered your question as to what determines the . As for twins/triplets, that's determined by the woman. For identical multiples, the mother ovulates one egg and it splits, forming 2 (or more) embryos, which are almost always the same (since 1 sperm created them all). However, very rarely, a mutation of the sperm's chromosome can mutate and form 2 or more different es. For fraternal twins, the mother ovulates 2 (or more) eggs and each egg is fertilized.