The genetic complexity of the R locus in maize : (gene complex, regulatory elements, paramutation)
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The evidence on the compound structure of the R locus is reviewed. The more recent analysis of another group of R accessions, collectively known as R-cherry, seems to indicate that this locus is a gene cluster separable into different discrete units. However, recombinational and mutational results appear to be in conflict with this interpretation. Different possible ways of reconciling these contrasting results are considered. The demonstration of the existence of an R-controlling element, accounting for the instability pattern of R-st, depends on the observation that the instability property of R-st is separable from the pigment determinant (Sc). This control element, (IR), exerts its action only in coupling with (Sc) ("cis -active"). Another R-st function, its capacity to induce paramutation, appears to be located at different positions in the R-st region and to be "trans-active".