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
Kaiser Wilhelm Institute of Anthropology, Human Heredity, and Eugenics its ideas in "Mein Kampf". He also authored "The Rehoboth Bastards and the Problem of Miscegenation among Humans" (1913) (), a field study which provided context for later racial debates, influenced German colonial legislation and provided scientific support for the Nuremberg laws. Under the Nazi regime, Fischer developed the physiological specifications used to determine racial origins and developed the so-called Fischer–Saller scale. He and his team experimented on Romani people and African-Germans, especially those from Namibia, taking blood and measuring skulls to find scientific validation for his theories. During World War II, the Institute regularly received human body parts, including