Two-Parameter Characterization of Chromosome-Scale Recombination Rate

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, $G= G_0 + k \cdot P$ 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 ($G_0$) indicates a relative excess of recombination on smaller chromosomes in a genome. Given $G_0$, the parameter $k$ predicts the increase of genetic map length over the increase of physical map length. The observed values of $G_0$ 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.