This chapter begins with single-gene Mendelian traits and compares them to quantitative traits where many genes are involved in determining phenotype. Quantitative trait loci (QTL) studies use molecular markers to pinpoint which sections of DNA are associated with variation in a trait like tomato fruit size. Some genes that impact quantitative traits have more effect on the trait than other genes. This activity explores the QTLs involved with color—a quantitative trait—in tomatoes by mapping the QTLs and then mapping mutant genes known to affect tomato color.