9.6 Meiosis increases genetic variation among offspring

• Objectives

• Describe how chromosome assortment during meiosis contributes to genetic variation.

• Explain how crossing over contributes to genetic variation.

• Compare and contrast mitosis and meiosis.

• Key Terms

• crossing over

• genetic recombination

• Genetic variety in offspring is the raw material for natural selection

• Assortments of ChromosomesMeiosis contributes to genetic variety.

• How the chromosomes in each homologous pair (tetrads) line up and separate at metaphase I is a matter of chance, like the flip of a coin.

• So, the assortment of chromosomes that end up in the resulting cells occurs randomly. In this example, four combinations are possible

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• If you know the haploid number for an organism, you can calculate the number of possible combinations in the gametes

• . The possible combinations are equal to 2n, where n is the haploid number.

• For the organism in Figure 9-18, n = 2, so the number of chromosome combinations is 22, or 4. For a human, n = 23, so there are 223, or about 8 million, possible chromosome combinations!

• Crossing OverThe number of different chromosome combinations in gametes is one factor that contributes to genetic variation.

• A second factor is crossing over—the exchange of genetic material between homologous chromosomes. This exchange occurs during prophase I of meiosis.

• When crossing over begins, homologous chromosomes are closely paired all along their lengths. There is a precise gene-by-gene alignment between adjacent chromatids of the two chromosomes.

• Segments of the two chromatids can be exchanged at one or more sites

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• Crossing over can produce a single chromosome that contains a new combination of genetic information from different parents, a result called genetic recombination.

• Review: Comparison of Mitosis and MeiosisYou have now learned about two versions of cell reproduction in eukaryotic organisms.

• Mitosis, which provides for growth, repair, and asexual reproduction, produces daughter cells that are genetically identical to the parent cell..

• Meiosis, which takes place in a subset of specialized cells in sexually reproducing organisms, yields four haploid daughter cells with only one set of homologous chromosomes. This set consists of one member of each homologous pair

• In both mitosis and meiosis, the chromosomes duplicate only once, in the preceding interphase.

• Mitosis involves one division of the genetic material in the nucleus, and it is usually accompanied by cytokinesis, producing two diploid cells.

• Meiosis involves two nuclear divisions, yielding four haploid cells.

• The key events that distinguish meiosis from mitosis occur during the stages of meiosis I..

• Mitosis and meiosis both make it possible for cells to inherit genetic information in the form of chromosome copies

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