12. Mutation of a to different alleles. The frequency of mutation of recessive a in the presence of Bt to the ap b allele as compared to the frequency to the A and Ab alleles can be ascertained by the classification of the aleurone dots into pale and deep colored. However in the aleurone it is impossible to differentiate between the A and Ab alleles and to determine the relative frequency of mutation to these two alleles it is necessary to test the relatively rare germinal mutations against the P gene. To date twelve germinal mutations giving deep colored aleurone and purple plants, with B Pl, have been tested. Eleven proved to be identical to the A allele while the remaining one gave brown pericarp. Since Ab produces a dominant brown pericarp it will be necessary to test this allele against A in order to find if the brown pericarp color is dominant to the red of A before one can draw the conclusion that it is a mutation to Ab. Irrespective of the outcome of this test it is an allele different from A and ap and mutations of a to three different alleles have occurred.
There are only two a alleles of different origin. Both of these are mutable in the presence of Dt. It is of some interest that on four occasions mutations of an a allele unstable with Dt have apparently occurred to an a allele which is stable with Dr. Stadler has found an a allele stable with Dt which arose as a mutation in his ultra-violet treatments.