| (2) Resistant x susceptible group, represented by two crosses, FKN crossed
with each of the two cultivars Tosson and (H. 62 x H. 722). The F1
plants in these two crosses were as resistant as the resistant parent, showing
complete dominance of resistance to stem rust. Results from F2
and F3 Showed that the resistant cultivar FKN might carry a dominant
gene (R) for resistance, while its allele (r) would be present
in the susceptible cultivars, Tosson and (H. 62 x H. 722). However, it was
previously detected that the dominant gene for susceptibility (S)
in the two susceptible cultivars was epistatic over (R1) in the cultivar
Kenya Farmer. However, gene (S) appeared from the results, to be
hypostatic to the gene (R) carried by FKN. (3) Resistant x moderately resistant crosses; represented by the cross FKN x Kenya Farmer. The F1 plants were as resistant as the resistant parent FKN, indicating complete dominance of resistance to stem rust. Results in F2 and F3 indicated the presence of two pairs of genes, including gene (R1) for moderate resistance in Kenya Farmer and gene (R) for resistance in FKN, as was previously assumed. Their recessive alleles (r and r1) gave susceptible plants. (4) Susceptible x susceptible group, represented by the cross Tosson x (H. 62 x H. 722). All F1, F2 populations, F3 families and backcrosses plants were as susceptible as both parents. These results showed clearly that the two susceptible parents possessed the same genetic constitution or possessed similar genes for susceptibility, which was previously suggested to be gene (S). For explaining the previous results obtained from the F1, F2, backcrosses and F3 of these groups of crosses on a factorial basis the following genes were suggested: R: dominant gene for resistance to stem rust, epistatic over gene (S) for susceptibility. R1: dominant gene for moderate resistance to stem rust. S: dominant gene for susceptibility, epistatic over gene (R1) for moderate resistance. r, r1, and S: recessive alleles giving susceptibility. According to these assumptions the genetic constitutions of the parents would be: Resistant: FKN RRr1r1ss Moderately resistant: Kenya Farmer rrR1R1ss Susceptible: Tosson and (H. 62 x H. 722) rr r1r1 SS. (Received February 19, 197l) |
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