Sex determination in humans happens at the time of conception itself. When a baby is conceived, a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell. This determines whether the baby will be genetically female or male. It is female if an X chromosome from the male parent has fused with the X chromosome of the female parent (XX). And, it is a male if the Y chromosome from the male parent has fused with the X chromosome of the female parent (XY).