Chromosomes are the structures that store genetic material in the form of DNA. During fertilization, when an egg and sperm cell join, the chromosomes from each cell combine to form a new set of chromosomes. This process is called mitosis. The chromosomes that are formed during mitosis are called homologous chromosomes, which are made up of two chromatids that are joined together at a point called the centromere. The chromatids hold the same genetic material, but may have different versions of the same gene. This allows for genetic variation in offspring.