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Meiotic recombination is essential for the production of viable haploid gametes in sexually-reproducing organisms, and creates new allelic combinations, which may facilitate evolution by natural selection and genetic drift (Barton & Charlesworth, 1998). Crossovers between homologous chromosomes establish physical connections (chiasmata) which enable proper chromosome segregation. In the absence of chiasmata, homologous chromosomes align as univalents on the metaphase plate and segregate randomly, leading to aneuploidy and inviable gametes; in mammals this also leads to pregnancy loss or severe mental and developmental defects in the surviving fetus. Proper meiotic chromosome disjunction is affected not only by the presence or absence of chiasmata, but also by their placement along the chromosome. Crossovers that are too distal or too proximal relative to the centromere contribute to …