For many years the principles of genetics have been applied to crop variety improvement with great success. Several crop species, notably corn, wheat, and tomato, have been used as model genetic systems because of their central importance to food production. Until recently, virtually all progress in both breeding and model genetic systems has relied on a phenotypic assay of genotype. Since the efficiency of a selection scheme or genetic analysis based on phenotype is a function of the heritability of the trait, factors like the environment, multigenic and quantitative inheritance, or partial and complete dominance often confound the expression of a genetic trait (33). Many of the complications of a phenotype-based assay can be mitigated through direct identification of genotype with a DNA-based diagnostic assay (5,7). For this reason, DNA-based genetic markers are being integrated into several genetic systems, and are expected to play an important role in the future of plant breeding.