The first source of genetic variation, independent assortment (Fig. 6), occurs in metaphase I, when the homologous chromosomes line up on the metaphase plate. Orientation of the pairs is random; therefore, which chromosome (maternal or paternal) goes to each daughter cell is random. A particular daughter cell has a 50% chance of getting either the maternal or paternal chromosome for each homologous pair. Figure 6 illustrates two pairs of homologous chromosomes and how their random alignment at the metaphase plate can lead to different combinations of chromosomes in the daughter cells that are produced. The chromosome pairs align independently of one another. Therefore, the number of different combinations of chromosomes in the daughter cells is equal to 2n, where n = the haploid number of chromosomes (or the number of chromosome pairs). In this example there are two chromosomes, so there are 22 = 4 different chromosome combinations in the daughter cells. If we look at the whole human genetic complement (n = 23), the number of different combinations is 223 = 8,388,608. Due only to the independent assortment of chromosomes in meiosis I, over 8 million genetically distinct gametes can be produced.
When making these computations, calculate the number of possibilities using the number of chromosome pairs, not the total number of chromosomes