Paran, I., Dept. of Plant Genetics and Breeding, Agricultural Research Organization, Volcani Center, PO Box 6, Bet Dagan 50250, Israel
Zamir, D., Dept. of Field and Vegetable Crops, Faculty of Agriculture, Hebrew University of Jerusalem, PO Box 12, Rehovot 76100, Israel

Phenotypic variation for quantitative traits results from segregation at multiple quantitative trait loci (QTL), the effects of which are modified by the internal and external environments. Because of their favorable genetic attributes (e.g. short generation time, large families and tolerance to inbreeding), plants are often used to test new concepts in quantitative trait analysis. Thus far, the molecular basis underlying allelic variation at QTL is similar to the identified variation for simple mendelian loci; namely, alterations in gene expression or protein function. Further comprehensive dissection of complex phenotypes will depend on our ability to link genetic components of the QTL variation to genomic databases.