Phenotype Dependence of the Combining Ability in Mutant Sugar Beet

Repetitive gametic selection for a higher frequency of the Adh1-Ssemilethal mutant allele of the alcohol dehydrogenase (ADH) gene yielded viable homozygotes Adh1-SS. The plants varied in phenotype from weak mutant to nearly normal (restored). Phenotypically different plants were individually tested for combining ability. This parameter was high in plants with the mutant phenotype and tended to decrease, rather than further increase, in plants with a restored normal phenotype. The results are discussed in terms of viability restoration mechanisms in homozygotes for semilethal mutant alleles.