Rice Genetics IV - IRRI books - International Rice Research Institute

traits of economic importance (Mackill 1999). In this chapter, we discuss the applicationof molecular markers to the mapping and manipulation of major genes in rice.Major- versus minor-gene traitsPlant traits can generally be classified as qualitative or quantitative. In the former, thephenotype of plants in segregating populations can be classified easily into discreteclasses, whereas, in the latter, the data usually show continuous variation. Qualitativetraits are almost always controlled by one or two major genes segregating in Mendelianfashion. Quantitative traits are usually controlled by several genes with smallereffects that result in a continuous variation in segregating populations. These traitsare typically sensitive to environmental factors, and the genetic effects are diluted bythis environmental variation and by the interaction between the two.In practice, there is a range of situations between the two extremes (Table 1). Forexample, some major genes are responsible for semiquantitative traits that segregatein a more or less discrete fashion. A common example is the semidwarfing gene suchas sd1 and the photoperiod-sensitivity gene Se1. In some cases, major genes can beresponsible for quantitative traits showing continuous segregation; an example is thesubmergence tolerance gene Sub1. In the mapping of quantitative trait loci (QTL), thepercent of the phenotypic variation under the control of each locus is generally usedto assess the effects of the specific locus on the trait. For the purposes of this discussion,we designate QTL responsible for more than 50% of the phenotypic variation as“major genes,” whereas loci controlling 25–50% of the variation would be “majorQTL.” For any particular QTL, the percent of the phenotype explained value willvary depending on the environment under which the trait is measured and the geneticpopulation used.Major-gene traits mapped in riceSince the first RFLP map of rice was constructed (McCouch et al 1988), hundreds ofgenes and QTL have been mapped. As QTL in rice has been discussed in severalprevious reviews (McCouch and Doerge 1995, Yano and Sasaki 1997) and in thisTable 1. Classification of major- and minor-gene traits.Percent ofTrait Segregation phenotypic variation Examples ClassificationexplainedQualitative Discrete 100 Purple leaf, blast resistance Major geneSemiquantitative Discrete 100 Semidwarfism, sd1 Major geneQuantitative Continuous >50 Submergence tolerance Major genegene, Sub1Quantitative Continuous 25–50 Stem rot resistance Major QTLQuantitative Continuous