Complex traits, including many quality and agronomic characteristics are under the control of multiple genes.
 

QTL mapping, a linkage map with DNA markers is developed based on a population segregating for the trait.
 

The trait/phenotype is dissected into discrete Quantitative Trait Loci.
 

The markers linked to these QTL are then used to track the desirable alleles through your breeding populations.
 

Successful projects often involve 4-6 QTL but have been carried out with as many as 12 QTL.