12.3The Purpose of Meiosis
Meiosis exists to provide gametes - the origins of next-generation progeny - with diverse gene combinations by mixing paternal and maternal genes. In this process, homologous chromosomes form pairs, which are distributed independently to separate gametes (Fig. 12-6A). Gametes can therefore have many combinations of homologous chromosomes. As an example, in humans (which have 23 pairs of homologous chromosomes), the number of the possible combinations is 223 (8.4 x 106). Genetic crossover also occurs between paired homologous chromosomes, resulting in gene recombination. Since crossover occurs independently in each sister chromatid, all four resultant chromatids have different gene combinations (Fig. 12-6B). In this way, new chromosomes with a mix of paternal and maternal chromosomes are created. In chromosomal recombination, the greater the distance between two genes, the more likely recombination is to occur between them. Recombination rarely takes place if two genes are close to each other. The distance between genes can therefore be estimated by measuring the gene recombination rate (known as genetic mapping).
As discussed above, intraspecies genetic diversity is increased during the meiotic process through the formation of many homologous-chromosome combinations and gene recombination by crossover. This diversity is believed to be advantageous in creating progeny that can expand its habitat to a variety of environments and adapt to rapidly changing circumstances.