The effect of gene conversion on intralocus associations

• Published in: Genetics (Austin) 1998-03, Vol.148 (3), p.1397-1399
• Main Authors: Andolfatto, P, Nordborg, M
• Format: Article
• Language: English
• Subjects: Animals; Chromosome Mapping; Crossing Over, Genetic; Gene Conversion; Humans; Letter; Models, Genetic; Selection, Genetic
• ISSN: 0016-6731; 1943-2631; 1943-2631
• DOI: 10.1093/genetics/148.3.1397

SEVERAL predictions of the genomic pattern of nucleotide polymorphism include the local recombination rate, r, as a parameter. In typical population genetic models, the parameter r, for two specified loci (or sites), is properly defined as the probability that a randomly selected gamete produced by a double heterozygote is a recombinant. In a given region, r can be measured directly through genetic crosses. This is typically done by observing markers spaced several centimorgans apart, in which case recombinants are produced almost exclusively by classical crossing-over. Over small intervals it may be assumed that crossing-over events are equally likely to occur at any point between two markers, and the probability of observing more than one can be neglected. Thus r, to a reasonable approximation, increases linearly over short physical distances. The problem we wish to draw attention to is that this model is often extrapolated to distances far too small to ignore the added effect of gene conversion events on the overall probability of producing a recombinant. In fungi and Drosophila, a common feature of models of homologous recombination is that Holiday junctions are resolved either as gene conversion alone or as gene conversion with the accompanying exchange of flanking markers. Thus some fraction of genetic exchanges will involve the transfer of short tracts of information from one gamete to another (i.e., gene conversion) without concurrent crossing-over. For clarity, we will refer to gene conversion without crossing-over as "gene conversion," and gene conversion accompanied by crossing-over as "crossing-over." Here we show that, at intragenic distances, gene conversion, rather than crossing-over, is likely to be the dominant force that breaks up associations among sites. We discuss implications for population genetic predictions for the behaviour of neutral sites closely linked to a site under balancing or directional selection.