39 Meiosis and Sexual Reproduction
Mechanisms that Increase Genetic Variation
The evolution of life on planet Earth is a dynamic process that is a direct result of genetic variation. Mutations in an organism's DNA produce changes in genes. When these changes occur, they may provide either a beneficial phenotypic change or a nonbeneficial one. Over time, processes such as natural selection favor organisms with mutations that are beneficial. If these changes provide enough of a benefit in the sexual reproduction of an organism, the mutations will become more prevalent in the population as a whole.
Figure 12: The sea anemone (order Actiniaria) reproduces via asexual reproduction.
In organisms like the sea anemone that reproduce without meiosis, the opportunity for introducing genetic variation into a population is very limited.
Courtesy of Laszlo Ilyes. Some rights reserved. 
One significant advantage for genetic variation produced by sexual reproduction over the consistency of asexual reproduction is seen with viral disease. In many species that produce asexually, such as the sea anemone, a single virus may have devastating effects on a population (Figure 12). In sexually reproducing organisms with a varied gene pool, a virus will likely have a less detrimental effect because some of the genetic variations that arise may provide some degree of resistance to the virus. In humans, for example, gene variants that confer resistance to viruses have probably been favored by natural selection. For example, a study in 2010 from the University of Milan analyzed 52 populations worldwide and identified 139 human genes that modulate susceptibility to viral infections.
Independent assortment produces new combinations of alleles.
How is genetic variation generated? There are several points during sexual reproduction at which genetic variation can increase. In meiosis I, crossing over during prophase and independent assortment during anaphase creates sets of chromosomes with new combinations of alleles. Genetic variation is also introduced by random fertilization of the gametes produced by meiosis. Any of the genetically unique sperm generated by a male may fertilize the genetically unique egg produced by a female.
During metaphase I, the homologous pairs of chromosomes are aligned along the metaphase plate. The orientation of the homologous pairs is random and is different for every cell that undergoes meiosis. In humans, this arrangement involves 23 different tetrads. Each tetrad contains one maternal and one paternal pair of sister chromatids. In one cell, for example, the tetrad corresponding to human chromosome 1 may align so that its paternal sister chromatids face toward one pole while the maternal sister chromatids are facing toward the other pole. And there is a 50% chance that the opposite orientation will occur during metaphase I in another cell.
Bioskill
Calculating the Probability of Genetic Variation
The number of different gamete types resulting from independent assortment of the homologs is calculated using the formula 2n where n = the number of chromosomes in a haploid cell of a given species. How many different combinations are possible in a fruit fly with 4 different chromosomes and a human with 23 different chromosomes?
In the fruit fly n = 4, so there are 24 different assortments possible = 2 x 2 x 2 x 2 = 16 possible chromosome combinations in the gametes.
In humans, however, where n is a much larger 23, there are far more possibilities. In fact, there are almost 8.4 million (223) different ways that maternal and paternal chromosomes could be assorted in gametes. This results in a massive amount of variation in the gametes that form at the end of meiosis II.
Bioskill
Crossing-over involves switching sections of DNA between two non-sister chromatids.
Recombinant chromosomes are made of DNA that has been randomly transferred between two non-sister chromatids of two homologous chromosomes. Crossing-over occurs early during prophase I while the homologous chromosome pairs begin to loosely bind to each other. While they are connected, the two non-sister chromatids switch sections of DNA at specific points. Usually one to three crossover events occur per chromosome depending on the size and species (Figure 13).
Figure 13: Crossing over during meiosis.
Click on the forward and back arrows to step through meiosis, selecting ‘with recombination’ or ‘without recombination’ to view the effect of recombination on the assortment of genetic information.
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Random fertilization increases genetic diversity.
When a male gamete and a female gamete finally meet, each is the result of an immense number of genetic possibilities created during independent assortment and crossing over. Human diploid cells have 23 pairs of chromosomes. Because of independent assortment during meiosis I, there are 223, or 8.4 million possible gametes that may be created even if crossing over didn't occur. In humans, there are about 2 to 3 crossovers per chromosome, and different crossovers occur in each meiotic division, so there is the potential to produce an enormous number of unique gametes.
Errors during meiosis.
The process of meiosis is highly regulated. Many different molecules and proteins are responsible for regulating the steps of meiosis as they occur. If any mistakes occur during the replication of chromosomes, it may have drastically detrimental effects on the resulting offspring. Errors may also occur during chromosome segregation. The term nondisjunction is used to describe the abnormal separation of chromosomes, resulting in the wrong number of chromosomes going to each gamete. There are many different human disorders that are the direct result of the incorrect separation and movement of homologous chromosome pairs. Disorders that are a direct result of nondisjunction include Down syndrome, Klinefelter syndrome, and XYY syndrome.