Meiosis is a special kind of cell division that produces gametes (sperm and egg). Prior to the initiation of this process, the genetic materials is in the form of indistinct threads called chromatins (see Figure 11-30), which condense to chromosomes at the time of cell division. At the beginning of meiosis in a cell of the ovary or testis, the randomly distributed chromosomes (from each parent) pair up, side by side. Each chromosome pair then copies itself exactly to make four strands linked by a centromere, which is shown in the diagram as homologous pair.
At crossover points, sections of a chromosome pair are cut and swapped. The crossover points can vary in each of the four strands. Once the genetic material is shuffled, pairs of chromosomes, now with new gene combinations, are pulled into two new nuclei as the cell divides into two (meiosis I). The gametes are produced by one more division (meiosis II). Each of the gamete contains only one copy of the newly shuffled gene.
Genetic variability depends on the different alignment of the chromosome pairs during the meiosis I division. The two newly shuffled versions can place themselves on either one of the daughter cells. There are 23 chromosome pairs in human cell, therefore they can line up in 223 = 8,388,608 different ways. Any one of a person's more than 8 million possible combinations of chromosomes can combine with any one of the more than 8 million combinations of his or her partner, raising potential variability to more than 70 trillion (8,388,6082) genetically unique individuals. Crossover (sometimes referred to as recombination) contributes even more variation.