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
Spann-Fischer Codependency Scale & Basha, 1996). The mean Spann-Fischer co-dependency score is approximated with a midpoint of 52.6, a "high" score of 67.2 and a "low" score of 37.3 suggested by Fischer, Spann, and Crawford (1991). No significant between-workshop differences were found for Spann-Fischer measures of co-dependency in the cases presented (F = .042, p = n.s.). Spann-Fischer Codependency Scale The Spann-Fischer Codependency Scale is a 16-item self-report instrument used to define and measure co-dependency in order to operationalize it as a personality disorder. Individual items are rated on a 6-point Likert scale, and then summed with two reversed items to describe co-dependency