18.3Significance of Meiosis
The significance of meiosis lies in providing gametes – the origins of next-generation progeny – with diverse gene combinations by mixing paternal and maternal genes. To create and transfer diverse combinations of DNA (genetic material) through the reproduction process is the key to ensuring the diversity of organisms.
In the meiotic process, homologous chromosomes form pairs, which are distributed independently to separate gametes (Figure 18-6A). Gametes can therefore have many combinations of homologous chromosomes (shuffling of chromosomes). The example shown in Figure 18-6A is based on the assumption that there are 3 pairs of homologous chromosomes. In this case, meiosis produces 23 or 8 types of gametes. For example, humans have 23 pairs of homologous chromosomes, and therefore the number of possible combinations is 223 (8.4 × 106). As mentioned earlier, genetic crossing-over occurs between paired homologous chromosomes, resulting in gene recombination. Because crossing-over occurs independently in each sister chromatid, all four resultant chromatids have different gene combinations (Figure 18-6B). In this way, through shuffling of chromosomes and chromosomal recombination, new sets of chromosomes with a mix of paternal and maternal chromosomes are created, generating diversity in gametes. Even if there are large numbers of sperm in the semen, none will be genetically identical to another.
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.
There are approximately 25,000 human genes. They are divided into 23 pairs of chromosomes, and shuffled through the reproduction process. Each chromosome will have approximately 1000 genes, so the genotypes and associated phenotypes on the same chromosome will be transferred as a set into the same gametes. This is called linkage. In some cases, however, genetic recombination can cause this linkage to break, in other words, the set of genes on the same chromosome can be separated and end up in different gametes.