Abstract
The Dt gene in maize, which is known only by its capacity to increase the mutability of the recessive a allele, was located by 4 point back-cross data. It is placed 7 cross-over units to the left of yg 2 in the short arm of chromosome 9 either in the thread-like chromatin strand or in the terminal knob. Examination of the published data on the genetic control of the mutation rate of the recessive a allele reveals that it cannot be accounted for in terms of factor interaction, epistasis and threshold values, but only by the mutable gene hypothesis.