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 R^r (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 R^g 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 (R^r → R^g) occur at appreciable rates in certain alleles, and the variation between R^r alleles in frequency of mutation to R^g appears to be uncorrelated with that of mutation to r^r. R (Cornell) is very low in frequency of mutation to R^g, 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 r^r and to R^g in the same plant (male germ cells) was tested extensively in 2 plants of R (Columbia), with the following results:

Mutations of R^r 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 B^w alleles occurring by mutation differ widely in level of action. In this respect B resembles A^b, which as previously reported mutates frequently to different levels of a^p type and rarely if ever mutates spontaneously to a.