Crossover interference

What is crossover interference? During prophase of meiosis in most eukaryotes, DNA recombination events between homologous chromosomes are induced to occur at high frequency, so there are usually multiple events per chromosome pair per meiosis. Whereas many of these recombination events are non-reciprocal, a subset results in reciprocal exchange of genetic material between chromosomes — crossovers, which in the presence of appropriate markers can be detected as linkage alterations. A genetic map is actually a map of the frequency and distribution of meiotic crossovers along a chromosome within a population. When researchers first began constructing genetic maps of Drosophila in the early part of the 20th century, they realized that the positions of multiple crossovers along a chromosome were not random with regard to each other. Muller observed that “the occurrence of one crossingover interferes with the coincident occurrence of another crossingover in the same pair of chromosomes, and I have accordingly termed this phenomenon ‘interference’”. Interference has subsequently been shown to operate in most — but not all — eukaryotes assayed. Interference results in widely spaced crossovers along chromosomes. Most eukaryotes average only a few crossovers per chromosome pair per meiosis. This means that interference can exert its effect across whole chromosomes (or chromosome arms). As chromosomes in many eukaryotes are large, interference must be able to act over megabase lengths of DNA. Indeed, in the nematode Caenorhabditis elegans, interference is capable of acting over a fusion chromosome of 50 Mb — nearly half the genome!