[Simple method of determining the R(xi) parameter of a half-chromatid chiasma]

Comparison of a point mutant of T4 phage with a deletion one in two-factor crosses against other point mutants demonstrated that at equal distances the deletion marker gave constantly lower frequencies of wild-type recombinants than the point marker did. The pattern of the distance dependence of this difference in the frequencies of recombinants agrees well with the hypothesis that mismatched regions formed by deletion markers undergo repair to the mutant allele with a probability close to unity. The equations have been obtained, connecting the frequency of the wild-type recombinants in a cross between two point mutants with the analogous frequency in a cross between a deletion marker and a point one. Analysis of the experimental data by means of these equations enabled the critical frequency R (xi) (recombination frequency between two point mutants separated by a distance equal to the mean length of hybrid region) to be measured in two independent ways.