Instability for Crown Rust Reaction and Kernel Color in F 6 ‐F 10 Lines of a Derived‐Tetraploid ✕ Avena sativa Hybrid 1

Summary Major genes governing dark kernel color and resistance to the crown rust fungus ( Puccinia coronata Cda. f. sp. avenae Fraser & Led.) were linked in four F 6 lines from the oat cross derived‐tetraploid C.I.7232 (2n‐28) ✕ Avena sativa L. var. ‘Clarion’ (2n=42). Most dark‐resistant (D‐R) plants were not true breeding; abnormal and irregular segregation ratios of D‐R and light‐susceptible (L‐S) plants occurred in F 6 ‐F 10 lines from D‐R parents. L‐S plants were highly self‐fertile and bred true. Cytological investigations revealed aneuploid plants (2n=43) in which the D‐R genes were carried on an extra chromosome and euploid plants (2n=42) in which the genes were in a closely paired bivalent. Differential gamete frequencies, certation, or nonviability of homozygous‐resistant genotypes due to abnormal dosage effects could account for the distorted ratios obtained and for the absence of stable, resistant lines.