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Sexual Reproduction and Variation

Biology (Year 12) - Variation and Mutation

Ben Whitten

The genetic component of variation comes down to the transferring of alleles, which are alternative forms of a gene. There are a few processes which occur during meiosis (sexual reproduction), which ultimately increase variation in the population. These processes include;

  1. Crossing over

  2. Independent assortment and random segregation

  3. Random fertilisation

Image: Synapsis and Crossing Over with Labels image, Image by Christinelmiller, Sourced Under a Creative Commons 4.0 License from Wiki Commons Crossing over occurs in meiosis I, in prophase. The homologous pairs of chromosomes (same chromosomes in pairs from mother and father) cross over with each other, exchanging chromosome segments. This recombination of chromosomes creates genetic diversity, as it allows for genes from mother and father to be exchanged which results in chromosomes with a different genetic makeup. The new combination of genes can lead to new traits being created in offspring. It is important to note, that multiple chiasma (where chromosomes cross over) can occur on the same homologous pair. The Law of Independent Assortment originates from mendelian genetics, which essentially refers to the random orientation of both maternal and paternal chromosomes on the equator during metaphase I. There are two possibilities in independent assortment; only paternal chromosomes are to the left, meaning that daughter cells have only paternal/maternal chromosomes each, or the lining up of chromosomes is varied leading to both paternal and maternal chromosomes in each gamete. The Law of Random Segregation refers to how in anaphase I, the maternal and paternal chromosomes are randomly lined up and move to opposite poles of the cell, meaning that random separation has occurred. Random fertilisation refers to the union of haploid gametes, egg and sperm, to create a diploid zygote, which leads to variation in the population. The union of two random gametes will create a new offspring with entirely new traits which haven't occurred before. When two gametes unite in fertilisation, the resulting cell has different combination of genes from either parent. Chromosomes exist in homologous pairs where the genes on one member of the pair control the same characteristics as the genes on the other pair. In the first meiotic division, the homologous pairs of chromosomes are pulled to each pole of the cell. When they seperate, they do it at random and independently of each other. Therefore when a sperm cell fertilises and egg, the resulting fertilised cell contains a combination of genes is arranged in an order that has probably never occurred. There are millions of sperm in a single mL of ejaculate, up to 100 million in fact! So, the release of 2-5 mL of semen means that there are up to 500 million sperm with unique genetic makeups which have the possibility of fertilising a female egg.

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