Selection of QTLs
QTL selection
Quantitative traits are often controlled by several to many genes. Theoretically, all markers in QTL regions could be used for MAS. However, some QTLs might only explain a very small portion of the total variation. It might not be economically feasible to include such QTLs in the breeding program. Breeders should focus on the QTLs that account for the largest proportion of phenotypic variance for the trait.
QTL validation
All QTLs selected for by MAS should be phenotypically expressed across environments and effective in different genetic backgrounds and in relevant growth stages of the crop. Therefore, we have to check whether the QTL found in the mapping population is also effective in other genetic backgrounds. To do this, first a NIL is developed containing only a single QTL. Then, this QTL-NIL is compared with the recurrent parent to see whether the QTL has the expected effect in the new genetic background. This is known as QTL validation.
Summary
→ In marker-assisted breeding, focus should be on QTLs with a major effect (and only after QTL validation).