From his lanky frame and other symptoms, Abraham Lincoln may have had Marfan syndrome. More than one pair of genes may interact to produce the phenotype. There may be more than two alleles for one locus, but each individual inherits only two alleles. The ABO system of human blood type involves three alleles. Aquatic environment influences the phenotype of water buttercup, Ranunculus peltatus.
Polygenic inheritance occurs when a trait is controlled by several allelic pairs at different loci.
Allelic pairs at different loci on a chromosome or on different chromosomes all control one trait. Gene alleles can be contributing or noncontributing. Contributing alleles have an additive effect, resulting in quantitative variations. Genes are located on chromosomes. Independently proposed in 1902 by Theodor Boveri and Walter S. Sutton.