In sexually reproducing organisms, three processes lead to most genetic variation: independent assortment of chromosomes in meiosis; crossing over of chromosomes in meiosis; and random fertilization. Each pair of homologous chromosomes consists of one chromosome inherited from the father and one from the mother. Here, we have color coded them blue and red. Each pair of chromosomes lines up independently of the other pairs in metaphase I of meiosis. Here you can see one of the possible arrangements and outcomes. Let's rearrange the chromosomes to get different arrangements and outcomes. There are two different ways that each chromosome pair can line up. That means that in the organism shown here, with a diploid number of four, independent assortment of chromosomes at metaphase I can produce gametes with four different combinations of maternal and paternal chromosomes. In a human being, with 46 chromosomes, more than eight million combinations are possible. 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's possible for a human being to produce an almost infinite variety of gametes. A sperm fertilizes an egg, producing a zygote. The random nature of fertilization adds to the variation arising from meiosis. Each parent is capable of producing a huge variety of genetically different gametes. The number of possible combinations among their offspring is staggering. Theoretically, one human couple is capable of conceiving a number of genetically different offspring that's far greater than the number of humans who have ever lived.