Recombination is the formation of new allele combinations in a gamete. It results from two events in meiosis, independent assortment and crossing over. Independent assortment occurs in meiosis I when each pair of homologous chromosomes lines up on the metaphase plate. Each pair lines up independently of other pairs. In each pair the paternal chromosome may be on the left or right. The number of possible combinations of maternal and paternal chromosomes in the nuclei produced by meiosis equals 2 raised to the power of n, where n is the number of pairs of chromosomes. For the 23 pairs of human chromosomes this amounts to over 8 million combinations.