Repeated Mutations in One Area of a Mouse Chromosome

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Abstract
Three balanced lethal lines of mice involve a dominant mutation, T, and a series of recessive mutations, tn, all of which are homozygous lethal and, when combined, T/tn, give a viable, tailless phenotype. The tn mutations are believed to be associated with an inverted section of chromosome IX since recombination is eliminated in this region. Among several thousand regular offspring from inbreeding the balanced lethal lines, exceptional normal-tailed types were found which contained the t-type mutation of the line in which they arose and another different mutation of the same type. 14 changes of the t type and 2 of the T type are estimated to have occurred. Cytogenetic studies are being made to determine whether the new t mutations are associated with new inversions or are new alleles within the same inversion. The t mutations have certain types of behavior in common with the concept of alleles, but differ in that the compounds containing two diff, alleles are viable, although the two component alleles are lethal when homozygous. It is suggested that the T locus in the mouse may belong in the middle of a continuous gradation of loci from those manifesting true allelism to those composed of several separable elements remaining together after adjacent duplication or repeat formation.