A change in the nucleotide sequence of a gene doesn't necessarily change the product of that gene. Structural mutations alter the positions of genes on chromosomes. Most new mutations are beneficial to the organisms that have them. Quantitative traits are affected by more genes than polymorphic traits. A single gene with two alleles will produce a continuous distribution in a trait.
Statistical methods reveal how many genes control variation in a trait.
A polymorphic trait arises when more than one gene affects it. Neither genetic polymorphism nor quantitative inheritance is affected by the number of genes involved. Dobzhansky and Ford were smarter than Morgan and Muller. It is easy to identify both homozygotes and heterozygotes. All plants and animals have them.