Mapping Functions in Tetrad and Recombinant Analysis

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Abstract
In the Ascomycetes and Basidiomycetes, all 4 products of a single meiosis can be isolated and characterized; analysis of segregation in such tetrads offers a valuable method for analysis of meiosis. Heretofore, methods for using tetrad data in mapping loci have been only approximate, especially for unordered tetrads. Perkins and Whitehouse have presented a method for determining distances of loci from their centromeres applicable when at least 3 non-linked loci are available. To compare map distances so obtained with those computed from recombinant frequencies it has been assumed by various authors that pab = 12PAB, where pab = recombinant frequency between loci A and B., and Pab = frequency of M II segregations yielding tetratype tetrads. This approximation can be valid only over short distances, for the limit of pab as the number of chiasmata between A and B increases is 1/2, whereas the limit of Pab is 2/3rds. An analytical solution to the problem leads to an equation expressing the exact relation between pAB and Pab: [image] Knowing Pab, the frequency of M II tetratype tetrads, the equation can be used to compute recombinant map values (pab) for values of Pab between the limits of 0 and 2/3. From the equation it can be shown that for values of Pab in excess of 0.2 the error introduced by use of the 1/2-approximation generally used becomes increasingly marked. Application of the equation is illustrated for cases in which a discrepancy between map distances calculated from recombinant and tetrad data has been noted. In cases in which the discrepancy is due to use of the 1/2-approximation, it is eliminated by use of the equation. In a more interesting type of case, the correction fails to eliminate the discrepancy, emphasizing the likelihood of a departure from randomness in chromatid crossing-over.
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