Meiosis and Genetic Variation Section 6.6

Objectives SWBAT describe how sexual reproduction creates unique gene combinations. SWBAT explain how crossing over during meiosis increases genetic diversity.

Vocabulary Independent assortment Recombination Crossing over Genetic linkage Genotype Phenotype

Sexual Reproduction and Unique Gene Combinations Main advantage of sexual reproduction – it gives rise to genetic variation within a species.

Meiosis and Genetic Variation Variation results from: The independent assortment of chromosomes during meiosis. The random fertilization of gametes. Independent Assortment: Since homologous chromosomes pair up randomly along the cell equator during meiosis 1, it’s a matter of chance which of the two chromosomes from any homologous pair ends up in a given gamete.

Meiosis and Genetic Variation In humans, there are 23 pairs of chromosomes and each pair lines up independently during meiosis 1. As a result, in one human sex cell there are approximately 8 million different possible combinations of chromosomes (223). Sexual reproduction, fertilization, produces offspring from the random combination of two gametes. In humans, the total number of possible chromosome combinations is more than 70 trillion (223 X 223).

Meiosis and Genetic Variation The possible number of chromosome combinations varies by species. For example, fruit flies have only 4 chromosomes – or 16 possible chromosome combinations in a sex cell. Since each egg and sperm has 16 possible chromosome combinations, the total number of possible combinations is 256 - 16 X 16.

Sexual Reproduction As the two examples (human and fruit fly) show, sexual reproduction creates unique combinations of genes. This results in organisms with unique phenotypes (an organisms physical characteristics). The offspring of sexual reproduction has a mixture of traits from both parents. Returning to our wolf example, one wolf sibling can be black while the other is gray.

Unique phenotypes from Sexual Reproduction Behavioral characteristics, not just appearance, are also part of an individual wolf’s phenotype. Possible behavioral characteristics for a wolf might be speed, ability to spot weakened prey; more or less aggressive, etc.

Crossing Over Independent assortment creates a lot of variation within a species. However, crossing over and recombination create even greater variation to that created by independent assortment. Creates new combinations of genes.

Crossing Over Crossing over - is the exchange of chromosome segments between homologous chromosomes during prophase 1 of meiosis 1. results in new combinations of genes

Crossing Over Once homologous chromosomes have paired with one another: Some of the chromatids are very clos to each other. One chromatid from each chromosome breaks off and reattaches to the other chromosome (there is a swap of DNA between chromatids). Crossing over (the swap of DNA) can occur multiple times within the same pair of homologous chromosomes. Crossing over happens any time there is a germ cell dividing.

Genetic Recombination The term recombination refers to any mixing of parental alleles. Therefore, crossing over is considered a recombination event.

Genetic Linkage Each gene has a specific location on a chromosome (we call this a gene’s locus). Some genes are close together (in terms of location) and will tend to be inherited together – this is known as Genetic Linkage. They will remain together during crossing over. Conversely, genes that are located far apart (or at least farther apart) are more likely to be separated when crossing over happens.

Genetic Linkage Genetic linkage allows scientists to measure the physical distance between two genes to be calculated. This has been important to creating genetic maps of many species.