Types of Crossing Over

Crossing over is classified by how many chiasmata form within a single tetrad, and consequently by how many of its four chromatids actually participate in an exchange. A single crossover involves the formation of just one chiasma and physically exchanges segments between only two of the four chromatids in the tetrad, leaving the other two chromatids in their original, parental configuration. A double crossover involves two chiasmata forming at different points along the same tetrad, and depending on exactly which chromatids participate at each of the two chiasmata, it can be a two-strand, three-strand, or four-strand double crossover, each of which restores or scrambles the allele combinations differently. A multiple crossover involves more than two chiasmata forming on the same tetrad; because each additional chiasma is progressively less likely, multiple crossovers occur at a very low frequency. This classification matters directly for calculating recombination frequency: a tetrad with no crossover at all contributes 0% recombinant chromatids, a single crossover or a three-strand double crossover contributes 50% recombinants (two of the four chromatids), a two-strand double crossover restores the original parental arrangement and so contributes 0% recombinants, and a four-strand double crossover, which is rare …