Position effect variegation has been studied in female mice heterozygous for the flecked X-autosome translocation, T(7; X)Ct. Some of these carried the spotting gene (s) which clarifies the variegated patterns. Others carried a second X-autosome translocation, T(X; 16)16H, which suppresses the randomness of X-chromosome activity.
It was found the position effect variegation stems primarily from early occurring events which lead to the formation of clones of cells with different phenotypes. In this respect the phenomenon appears to parallel that found in Drosophila. However, in the mouse, late-occurring events are also found which can only be readily accounted for by the reactivation of previously inactive loci. They occur, not only during foetal development, but throughout the life-time of the animals and in a manner which suggests they derive from a progressive retreat of the inactivating influence of the heterochromatic X chromosome back along the attached autosome towards the breakpoint. It is proposed that the early occurring events do not lay down fixed programmes of gene suppression, as proposed for Drosophila, but that, like the later-occurring events, they represent the reactivation of previously inactivated loci. The possibility that this might also be true for Drosophila is discussed.
The study also provided evidence favouring the view that the X-chromosome controlling element, Xce, modifies the heterozygous phenotypes of X-linked genes by biasing the randomness of the X-inactivation process, rather than by operating through cell selection mechanisms. The data also support and extend Mintz's (1967) concept of pigment pattern differentiation.