Luis's wife tells him frequently that their baby looks a lot like him. Luis jokingly says, "It's all in the excellent Alvarez genetic code—we are all good-looking in my family!" Just where does the Alvarez family's genetic code come from? All cells, with the exception of mature sex cells, have 46 chromosomes—forming 23 pairs. Chromosomes are rodlike structures within the nucleus. Egg and sperm cells have half that number; that way, during fertilization, the mature sex cells unite and each contributes 23 chromosomes to the embryo. This means, of course, that Luis's baby has fully half of his wife's genetic code as well.