VI.2.2.1 In gene conversion, one allele is converted into another allele.
In gene conversion, one allele is converted into another allele. This broadly conceived definition would also encompass ordinary mutations andrecombinations. However, gene conversion is generally understood in a somewhat narrower sense, as a process in which the sequence of some DNA segment in the cell is corrected according to the sequence of some other segment located either at some other locus or, more frequently, in the same locus on a homologous chromosome (Fig. VI.4).
Gene conversions in which the sequence of one allele of a certain gene is corrected according to the sequence of another allele in the same gene located on a homologous chromosome are of great importance in the evolution of diploid organisms. This conversion can be highly asymmetric and the probability of correction of allele A according to allele B can be many times greater than the probability of the opposite process (Lamb 1998). Asymmetry can occur in various ways. For example, the probability of the two types of conversion can be identical, but allele A can more readily undergo mutation processes. In repairing the damaged locus, the repair apparatus of the cell will use the DNA segment on the homologous chromosome; however, this segment can bear the other allele of the particular gene.
Gene conversion that occurs between sequences in different loci can play an important role in evolution. If longer segments with a similar sequence of nucleotides are located in the genome at various loci, for example, multiple copies of genes for ribosomal RNA, then gene conversion can also occur and frequently does occur between them (Murti, Bumbulis, & Schimenti 1994). Sequences that best act as a template in gene conversion can gradually proliferate to many loci in the genome in this way – leading to homogenization of the given sequences.
Molecular studies performed on a number of model organisms have demonstrated that gene conversion is a very frequent phenomenon and affects most genes. At the present time, it seems that its existence follows directly from the crossing-over mechanism and necessarily always accompanies it (Otto & Feldman 1997). However, other authors have primarily pointed out that the frequency of gene conversion is much greater than that of crossing-over and that some other molecular mechanism must also be responsible for it (Storlazzi et al. 1995).