During prophase I stage of meiosis, homologous chromosomes form groups of four called tetrads. Chromosomes in the center overlap and pieces of DNA are exchanged during synapsis, in a process known as crossing over. This occurs between non-sister chromatids. The arms of two nonsister chromatids can overlap forming chiasma. At these sites of overlap, chromosome segments are exchanged or genes can be exchanged. It reshuffles the genetic information further. It is estimated that 80 to the 23rd power is the variability produced by a couple. Another process is random assortment – there are 2 to the n power combinations(2 represents mother and father) and since there are 23 chromosomes, it is 2 to the 23rd power or 8,388,608 combinations of the maternal and paternal DNA. It is almost impossible to have any two children genetically identical aside from identical twins. Because this occurs, much like shuffling a deck of cards, new combinations of genes are possible that will be enclosed within the gametes--sperms and eggs. Gene recombination occurs because loci on different chromosomes assort independently of one another when gametes are formed. This too results in many different combinations of genes in gametes.