Genetical analysis of components of overall plant shape.

The genetical control of six characters, which were taken as jointly reflecting the overall shape of the plant, was analysed using four true-breeding lines of Nicotiana rustica. F1 F2 and first backcross generations were raised from all of the possible pairwise combinations between the lines. The particular relationships between the lines provided a basis for the analysis which was an extension of the normal model fitting procedures described by Mather and Jinks (1971).The first step in the analysis was to test whether the allelic differences present between the inbred lines p1 and P5 had been maintained in the two lines B2 and B35, derived from an earlier cross between the former. If the allelic differences between p1 and P5 were present between B2 and B35, it was possible to proceed straight-forwardly by fitting a model consisting of m, two symmetrical [d]'s and the relevant non-additive parameters. If B2 and B35 were homozygous for the same alleles at loci by which p1 and P5 differed, in other words if significant asymmetry in the gene distributions was present, the model had to be extended to cover the effects of such genes.All six characters investigated were shown to be subject to genetical variation. From the composition of the genetical models that were necessary to account for the observations from each of the characters, it was inferred that they should be amenable to at least partially independent adjustment by selection.