Most traits that plant breeders wish to improve are quantitative, rather than qualitative, in nature. Quantitative traits vary continuously (e.g., yield, quality, stress tolerance etc.), whereas qualitative ones are typically (not always) binary (yes vs no - e.g., resistance to a fungus, colour of flower etc.). Quantitative traits are usually governed by a number of genes, while qualitative ones are often simply inherited (one or two genes). The loci involved in the inheritance of quantitative traits are commonly called QTL (quantitative trait loci).
There may be many regions in a genome affecting a particular trait. For some traits, as many as 50 QTL may be identified, but usually most of these are of too small effect to be useful in MAB; typically the number of useful QTL is less than 10.