A Mouse Translocation Suppressing Sex-Linked Variegation

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Abstract
Female mice heterozygous for the sex-linked gene tabby (To) normally show a striped or variegated coat pattern resulting from random inactivation of one or other X-chromosome. In a stock descended from an irradiated female this variegation did not occur and heterozygotes for tabby looked wild-type. This led to the hypothesis that the wild-type allele of Ta had been translocated to an autosome and so was remaining active in all cells; the presence of a reciprocal X-autosome translocation was then demonstrated cytologically. A similar suppression of variegation was found for two other sex-linked genes, bent-tail (Bn) and blotchy {Bld), and also for both tabby and blotchy in crossover animals which had the mutant alleles on their translocated X-chromosomes. In these animals the mutant allele was fully expressed and the wild-type allele not at all. This showed that, in addition to the translocated X being active, the normal X was inactive in all cells, and demonstrated that the variegated effect in normal heterozygous females depended on the randomness of X-inactivation. Linkage tests suggested that the order of loci in the X chromosome was Bn-bteak-Ta-Blo, but this was not firmly established. The autosome involved in the translocation has not yet been identified.