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What are the events that lead to genetic variation in gametes?

Sources of genetic variation consist of random fertilisation and mutation. Mutation is a product of mistakes made during DNA replication. There are two main types of mutation; point gene mutation and chromosomal mutation.

Point gene mutation consists of three subtypes; base pair substitution, deletion and insertion. The latter two result in frameshift and therefore can have a much larger effect on the function of genes than substitution which only effects one base pair.

Chromosomal mutations can occur in 5 subtypes; deletions of segments of a chromosome, duplications, inversions, translocations and polyploidy. Duplications can be advantageous to a species and occur as a result of unequal crossingover during meiosis resulting in more than one segment of a chromosome present in a single set of chromosomes. Inversions are rotations of a segment and can either include the centromere (pericentric) or exclude it (paricentric). Translocations are the movements of a segment either within the same chromosome (which is known as a transposition) or exchanges across different chromosomes. This alters the patterns of linkage (when two genes tend to stay together even when crossingover occurs) and is therefore often deleterious. NB translocation occurs between nonhomologous chromosomes, but crossingover occurs between homologous chromosomes. Polyploidy is a change in the number of sets of chromosomes and can occur via two routes; a mistake in meiosis where the final devision leading to 4 haploid cells doesn't occur and instead produces diploid cells, these can be self fertilised (autopolyploidy) resulting in triploidy (three sets of chromosomes as long as the species orininally had two sets of chromosomes), or allopolyploidy which is a result of cross-breeding between two different species.

Furthermore, meiosis is another event that leads to genetic variation via genetic recombination. There can also be crossingover between nonsister chromatids in prophase I, followed by independent assortment in metaphase. In independent assortment, homologous chromosomes aline along the equator of the cell. The maternal and paternal chromosomes can either be arranged separately on the two sides of the equator or jumbled up with some maternal and paternal chromosomes on the same side. Differences in this alignment will result in gametes with different genetic code being produced. 

Danielle W. GCSE Biology tutor, A Level Biology tutor

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