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Genetic consequence of meiosis

Humans and other complex multicellular organisms reproduce sexually because this ensures genetic variation in the population. By producing gametes through meiosis rather than mitosis, the offspring inherits the correct number of chromosomes once the sperm and egg fuse during fertilization. It also ensures that each gamete is unique.

Each gamete is genetically unique because of the random assortment of paternal and maternal homologs and because of the recombination of maternal and paternal segments of chromosomes—with their sets of genes—that occurs during crossover (Figure 1).

Four possibilities of chromosome arrangement with the set of two chromosomes are shown. The first possibility shows the metaphase one and two resulting in four gametes, two of them having two paternal chromosomes, named generic arrangement 1, and the other two having two maternal chromosomes, named generic arrangement 2. The second possibility shows the metaphase one and two resulting in four gametes, where two of them have first maternal and second paternal chromosome, named generic arrangement 3, and other two have first paternal and second maternal chromosome, named generic arrangement 4.

Figure 1. Random Assortment of paternal and maternal homologs.

Let’s use an example image with two chromosomes (n=2) undergoing independent assortment at metaphase I1. In this case, there are two possible arrangements, the paternal and maternal chromosomes could separate independently as seen in the upper panel, or the maternal and paternal chromosomes could mix as shown in the lower panel. This ultimately means that there are four possible arrangements. Now imagine how many possibilities there are with 23 chromosomes (n=23).

To demonstrate randomness, independent assortment at metaphase I1, consider a cell with n = 2. In this case, there are two possible arrangements at the equatorial plane in metaphase I1, as shown in the upper cell of each panel. These two possible orientations lead to the production of genetically different gametes. With more chromosomes, the number of possible arrangements increases dramatically.