The self-incompatible composite Senecio Jacobaea (ragwort) exhibits geographic variation in the frequency of rayed and discoid (rayless) individuals. Hybrid progenies from within- and between-morph crosses were established in a seminatural (garden) environment to determine whether patterns of segregation conform to single-gene predictions (as found in other Senecio species), whether the direction of dominance is conducive to rapid evolutionary change in ray morphology, and whether geographically distant populations of the discoid morph utilize the same or different genes to suppress ray development. Data from segregating F-2 and BC families were consistent with a genetic model involving one major locus and an unknown number of modifiers. Analysis of F-1 progenies from different intermorph crosses using the same rayed plant as a seed parent revealed a variable and incomplete pattern of dominance, with a trend toward partial dominance in some crosses. Hybridizations between discoid populations produced a few rayed progeny (4%), but there was no tendency for the frequency of rayed progeny to increase with the geographic distance separating the parent populations. Results of this study indicate that major mutations have been important for the evolution of discoid populations of ragwort, that ray-suppressing mutations should be directly available to selection in most populations, and that the suppression of rays is conditioned by the same or similar gene(s) in Atlantic and Baltic populations of the discoid taxon.