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Meiosis and Genetic Variation

Section 6.6

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Objectives

• SWBAT describe how sexual reproduction creates unique gene combinations.

• SWBAT explain how crossing over during meiosis increases genetic diversity.

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Vocabulary

• Crossing over• Recombination• Genetic linkage • Independent assortment• Genotype • Phenotype

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Sexual Reproduction and Unique Gene Combinations

• Main advantage of sexual reproduction – it gives rise to genetic variation within a species.

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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.

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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).

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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.

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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.

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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.

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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.

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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

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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.

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Recombination

• The term recombination refers to any mixing of parental alleles. – Therefore, crossing over is considered a

recombination event.

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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.

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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.