3. Spontaneous Mutation. The frequency of spontaneous mutation to colorless aleurone types varies widely in different R alleles. The most mutable of the alleles studied is Rr (Cornell), which yields r mutations at the rate of about 2 per 1000 gametes. At the other extreme are a few alleles which give no mutations in populations of 25,000 to 100,000 gametes.
As previously reported, differences in mutability are inherent in the gene itself, since they are maintained when a highly mutable and a rarely mutable allele are combined in a heterozygote, so that the mutations must occur in precisely comparable cells. This comparison is made possible by the fact that the mutations affecting aleurone color do not affect plant color, and in a heterozygote R1 R2, in which plant color is distinct in the two alleles combined, the identity of the gene mutating is readily determined. For example, when R (Cornell) is combined with an Rg of low mutability, the mutants produced by the F1 plants are almost exclusively r (Cornell).
In addition, however, there is a pronounced effect of modifiers upon the frequency of R → r mutation. Homozygous R (Cornell) stocks extracted from crosses of the type mentioned show lowered mutation rates, in some cases much lowered. Different homozygous strains extracted from the same F1 plant show distinctly different rates.
Mutations to colorless plant types (Rr → Rg) occur at appreciable rates in certain alleles, and the variation between Rr alleles in frequency of mutation to Rg appears to be uncorrelated with that of mutation to rr. R (Cornell) is very low in frequency of mutation to Rg, while certain other A alleles yield plant color mutations at moderately high rates, none however approaching the frequency of aleurone color mutations in R (Cornell). The frequency of mutation to rr and to Rg in the same plant (male germ cells) was tested extensively in 2 plants of R (Columbia), with the following results:
Mutations of Rr to intermediate levels appear to be very rare. On the contrary B mutates frequently to intermediate levels, and no mutations of B to b have been found. The Bw alleles occurring by mutation differ widely in level of action. In this respect B resembles Ab, which as previously reported mutates frequently to different levels of ap type and rarely if ever mutates spontaneously to a.