2. Goldschmidt in the Proc. Nat. Acad. Sci. 1943 reports a situation in Drosophila melanogaster where the interaction of alleles at two different loci gives results somewhat similar to those reported for unstable genes. He suggests that the idea of unstable genes be abandoned, and that the so-called unstable genes of Drosophila and maize can be accounted for in terms of factor interaction, apistasis, and threshold conditions. He specifically cites the a-Dt case in maize. According to his interpretation the apparent mutations of a to A, believed to be induced by the Dt gone, are in reality cases where a new Dt allele (which will be represented Dt^A) produces the color ascribed to the A allele. He also states that no published data exist which negate his interpretation. Actually two decisive experiments have been published which establish the correctness of the mutation hypothesis. (1) The A alleles obtained by mutation from recessive a show the expected linkages with genes in chromosome 3. On Goldschmidt's scheme the color-producing allele would be in chromosome 9 since Dt is in that chromosome. (2) When a mutation of a to A occurs in a cell of a a Dt Dt constitution the constitution of that cell following mutation is A a Dt Dt. On Goldschmidt's scheme it should be a a Dt^A Dt.-

M. M. Rhoades-