Two-Parameter Characterization of Chromosome-Scale Recombination Rate

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The genome-wide recombination rate (RR) of a species is often described by one parameter, the ratio between total genetic map length (G) and physical map length (P), measured in centiMorgans per Megabase (cM/Mb). The value of this parameter varies greatly between species, but the cause for these differences is not entirely clear. A constraining factor of overall RR in a species, which may cause increased RR for smaller chromosomes, is the requirement of at least one chiasma per chromosome (or chromosome-arm) per meiosis. In the present study, we quantify the relative excess of recombination events on smaller chromosomes by a linear regression model, which relates the genetic length of chromosomes to their physical length. We find for several species that the two-parameter regression, G= G₀ + k · P provides a better characterization of the relationship between genetic and physical map length than the one-parameter regression that runs through the origin. A non-zero intercept (G₀) indicates a relative excess of recombination on smaller chromosomes in a genome. Given G₀, the parameter k predicts the increase of genetic map length over the increase of physical map length. The observed values of G₀ have a similar magnitude for diverse species, whereas k varies by two orders of magnitude. The implications of this strategy for the genetic maps of human, mouse, rat, chicken, honeybee, worm and yeast are discussed.