Allelic Recombination and Its Relation

N recent years a number of examples have been reported in which crosses Ibetween allelic mutants of Saccharomyces cerevisim (ROMAN 1956a; ROMAN and JACOB 1958) produce nonmutant progeny with a frequency which is higher than is expected from reverse mutation. Such revertants can be shown to result from an event which does not have the characteristics of crossing over. ROMAN and JACOB (1958) have described one such case in mitotically dividing diploid yeast cells involving two allelic isoleucine-requiring mutants of different origin. This system has been examined further in the present study by including two more allelic mutants at this locus. The relationship between allelic recombination and the recombination of outside markers among spontaneous revertants has been examined in detail and the mutant sites of the four alleles have been ordered on the chromosome.