A second source of genetic variation during meiosis is the exchange of genetic material between the maternal and paternal chromosomes, a process called crossing over or recombination (Fig. 7). While the homologous chromosomes are paired together in prophase I, pieces of one chromosome may be exchanged with the identical portion of the other chromosome. This means that the resulting chromosomes are not entirely maternal or paternal, but rather a mixture of both. In humans, crossing over occurs about 2-3 times per chromosome pair, between nonsister chromatids only (not between sister chromatids).