1. 9.6 Meiosis increases genetic variation among offspring

2. 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.

3. Key Terms
crossing over
genetic recombination

4. Genetic variety in offspring is the raw material for natural selection

5. Assortments of Chromosomes Meiosis 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.

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

8. 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!

9. Crossing Over The 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.

10. 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

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

13. Review: Comparison of Mitosis and Meiosis You 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..

14. 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

15. 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.

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

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