In 1909 Swedish botanist Nils Herman Nilsson-Ehle, a professor at Lund University, advanced the "multiple factor" theory, or theory of polygenic inheritance, in which a trait is produced from the cumulative effects of more than one gene. Traits that display a continuous distribution, such as height, hair or skin color, are polygenic. The inheritance of polygenic traits does not show the phenotypic ratios characteristic of Mendelian inheritance, though each of the genes contributing to the trait are inherited as described by Mendel. Einvironmental factors may affect polygenic inheritance, thus adding still other contributing factors to the "multiple factor" theory.
Nilsson-Ehle, "Kreuzungsuntersuchungen an Hafer und Weizen," Lunds Univiversitets Årsskrift. N.F. Atd 2, 5, Nr. 2 (1909) 1-122, N.F. Afd. 2, 7 (1911) Nr. 6, 1-84.
♦ Independently of Nilsson-Ehle, in 1910 American plant geneticist Edward Murray East of Harvard University published an essentially identical theory in "A Mendelian Interpretation of Variation that is Apparently Continuous," American Naturalist 44 (1910) 65-82.
J. Norman (ed) Morton's Medical Bibliography 5th ed (1991) nos. 245, 245.1.