9.6 Meiosis increases genetic variation among offspring
Transcription
9.6 Meiosis increases genetic variation among offspring
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 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. • So, the assortment of chromosomes that end up in the resulting cells occurs randomly. In this example, four combinations are possible • • 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 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. • When crossing over begins, homologous chromosomes are closely paired all along their lengths. There is a precise gene-bygene alignment between adjacent chromatids of the two chromosomes. • Segments of the two chromatids can be exchanged at one or more sites • • 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 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.. • 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 •