Human hair color account for all possible shades of brown, blond, or red (for example, platinum blond versus dark blond/light brown), nor does it explain why hair color sometimes darkens as a person ages. Several gene pairs control the light versus dark hair color in a cumulative effect. A person's genotype for a multifactorial trait can interact with the environment to produce varying phenotypes (see quantitative trait locus). Natural hair color can be brown, blond, black, red, or white; all natural hair colors including grey, white and lightest blond, are shades of brown. The Fischer–Saller scale, named after Eugen Fischer and , is
Eugen Fischer in his manifesto, "Mein Kampf" ("My Struggle"). Fischer was born in Karlsruhe, Grand Duchy of Baden, in 1874. He studied medicine, folkloristics, history, anatomy, and anthropology in Berlin, Freiburg and Munich. In 1918, he joined the Anatomical Institute in Freiburg in 1918, part of the University of Freiburg. In 1927, Fischer became the director of the Kaiser Wilhelm Institute of Anthropology, Human Heredity, and Eugenics (KWI-A), a role for which he'd been recommended the prior year by Erwin Baur. In 1933 Fischer signed the "Loyalty Oath of German Professors to Adolf Hitler and the National Socialist State". In 1933, Adolf