Now, let's look at how crossing over creates even more genetic variability. During prophase I of meiosis, homologous chromosomes pair up very closely, and corresponding parts of two nonsister chromatids may trade places. This process of crossing over creates variation by producing chromosomes that combine the genes inherited from two parents. Here, the process produced a total of four genetically different gametes. See the different outcomes of crossing over. In humans, crossover events happen an average of two or three times per chromosome pair, greatly increasing the variation among eggs and sperm. Note that crossing over produces some parental gametes with chromosomes like those of the parents, and some recombinant gametes with a mixture of genes from both sets of chromosomes. Independent assortment and crossing over occur simultaneously during meiosis, multiplying the number of genetic variations among gametes. Observe the combined effects of independent assortment and crossing over. Because each pair of chromosomes lines up independently, and crossovers can occur almost anywhere along each pair of chromosomes, it is possible for a human being to produce an almost infinite variety of gametes.