Mixed inheritance model for resistance to agromyzid beanfly (Melanagromyza sojae Zehntner) in soybean

A quantitative trait could be controlled by a few major genes and many polygenes. Distinguishing the effects of major genes from polygenes and/or environments is important for understanding the expression of a major gene in relation to its genetic background, and for predicting the segregation of a cross in breeding. Our objective was to re-analyze the resistance of soybean to agromyzid beanfly by a mixed inheritance model. Number of insects in stem (NIS) was used as an indicator of resistance. The previous result from the segregation ratio of resistance and susceptibility was that resistance was controlled by one dominant gene. The major results from the mixed inheritance model were (1) the inheritance of resistance was controlled by one major gene along with minor genes; (2) Additive and dominance effects of minor genes were generally less than those of the major gene and varied among crosses, indicating different minor gene systems; (3) Heritability was higher for the major gene than for the minor genes; (4) The F-2 plants and F-2:3 lines were classified into appropriate genotypes according to their posterior probabilities and the critical value to distinguish resistant and susceptible plants was given for NIS based on the classification. These results indicated that mixed major gene and polygene genetic analysis was superior to the frequently used classical Mendelian method.