hereditary Variation

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What helps ensure the survival of a species?

Genetic variation. It is this variation that is the essence of evolution. Without hereditary differences amongst individuals, "survival of the fittest" would not it is in likely. Either every survive, or all perish.

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Crossing-Over

Crossing-over occurs during prophase I, and it is the exchange of hereditary material between non-sister chromatids that homologous chromosomes. Recall during prophase I, homologous chromosomes line up in pairs, gene-for-gene under their whole length, creating a configuration with four chromatids, well-known as a tetrad. In ~ this point, the chromatids are very close to each other and some product from 2 chromatids switch chromosomes, that is, the product breaks off and also reattaches at the same position on the homologous chromosome (Figure below). This exchange of genetic material can happen many times in ~ the very same pair of homologous chromosomes, creating unique combine of genes. This process is additionally known as recombination.

Crossing-over in meiosisHow To Find The 3Rd Angle Of A Triangle, How To Find The Third Angle Of A Triangle

In other words, every human pair could create a kid with end 64 trillion unique chromosome combinations!

See How cells Divide: Mitosis vs. Meiosis at http://www.pbs.org/wgbh/nova/miracle/divide.html for an computer animation comparing the 2 processes.