Gene conversion

Gene conversion is a type of recombination, distinct from chromosomal crossover, where the DNA sequence of one chromosome changes the DNA sequence of another without any effect on its own sequence.

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Gene conversion occurs because any of the four strands on one chromosome can associate with any of the four strands on its partner, especially in regions where there is a high level of sequence homology, and mismatch repair will alter the sequence of one of the strands. This can transform one allele into a duplicate of another, and is one cause of non-Mendelian inheritance.

Normally, an organism that has inherited different copies of a gene from each of its parents is called heterozygous. This is generically represented as genotype: Aa (i.e. one copy of variant (allele) 'A', and one copy of allele 'a'). When a heterozygote creates gametes by meiosis, the alleles normally split, and end up in a 1:1 ratio in the resulting cells. However, in gene conversion, a ratio other than the expected 1A:1a is observed, in which A and a are the two alleles. Such examples are 3A:1a, 1A:3a, 5A:3a or 3A:5a. In other words there can, for example, be three times as many A alleles as a alleles expressed in the daughter cells, as is the case in 3A:1a.

In population genetics, gene conversion acts to homogenise the DNA sequences composing the gene pool of a species. Every gene conversion event takes two DNA sequences that are homologous but not identical, because of sequence mismatches, and yields two identical DNA sequences. Gene conversion links DNA sequences between different organisms of a species. Over time, gene conversion events yield a homogenous set of DNA sequences, both for allelic forms of a gene and for multi gene families. Interspersed repeats act to block gene conversion events, thus catalysing evolution of new genes and species.