Compound loci and coincident mutation inNeurospora

Summary To explain the high frequency of partial and intermediate back-mutants and the back-mutation of deletions, a model of mutation production based on unequal sister strand exchange in multi-repeat loci is proposed.The recovery of complementary products of mutation which the model predicts should be produced from single nuclei is cited as evidence for its plausability.Back-mutation of the mutant used (ahis-3 mutant,Gg59) is associated with a high frequency of simultaneous switch mutations of other characters in the same genome.It is suggested that unequal sister strand exchange may also contribute to the production of spontaneous mutations.The validity of citing ability to back-mutate as critical evidence for classifying a mutant as a point (single base pair substitution?) mutation is queried.With 2 Figures in the Text