Mode of inheritance in fruit acidity in apple analysed with a mixed model of a major gene and polygenes using large complex pedigree

Fruit acidity is an important characteristic to determine the marketability of apple (Malus x domestica Borkh.). To reveal the mode of inheritance in fruit acidity and to estimate the genetic parameters, we performed segregation analysis using a population from an apple breeding programme. Four models (mixed, Mendelian, polygenic and environmental) were compared to find the most likely mode of the inheritance of acidity. The phenotypic variance of acidity in the population was properly explained using a mixed model of a major gene and polygenes. The genotypic values of the major gene (AA, Aa and aa) were estimated to be 0.45, 0.52 and 0.92 g/100 ml in titratable acidity, respectively. The values of the homozygote (AA) and the heterozygote (Aa) were very close and lower than that of the other homozygote (aa), indicating that an allele of the major gene appeared to have complete dominance with a function of lowering acidity. The estimate of the heritability after accounting for the major gene was moderately high, 0.43, in the mixed model. This means that even with removal of the effect of the major gene, acidity could fluctuate considerably by the effect of polygenes. The proportions of progeny with adequate acidity differed greatly, depending on which genotype was used as parents for the crossing. Therefore, it is very important to know the genotypes of parental cultivars before crossing.