Mitotic crossover is a natural occurrence during the creation of egg and sperm cells where a pair of chromosomes will switch genes between them before the new cell is created. As a result, the two chromosome created will have mixed genes from both of the person's parents and not only chromosomes that are identical to each parent as would be expected. Although first identified in fruit flies in 1936, it is now known to occur in most life forms that use sexual reproduction, including humans.