Assuming you ' re looking for a quick explanation to tack on to the one you already gave your kidSperm and ova are haploid that is , they only have half the chromosomes that the body has , one chromosome for each chromosome pair , so 23 chromosomes in the sperm and 23 in the egg . Those 46 chromosomes determine everything about the baby . Eye color , hair color , skin tone , etc . The very last pair are the sex chromosomes . The one from the ova is always an X . The one from the sperm can be an X or a Y . If the baby gets a Y from the father , then they will be a boy . If the baby gets an X , then they will be a girl . Since there are a ridiculously large number of individual sperm cells in the volume of one ejaculate , neither parent really chooses , but since the Y or X possibility can only come from the sperm , if you had to assign a responsible party , it ' d be the father