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Melanin comes in two types: pheomelanin (red) and eumelanin (very dark brown). Both amount and type are determined by four to six genes which operate under incomplete dominance. One copy of each of those genes is inherited from each parent. Each gene comes in several alleles, resulting in a great variety of different skin tones.
Incomplete dominanceDiscovered by Karl Correns, incomplete dominance (sometimes called partial dominance) is a heterozygous genotype that creates an intermediate phenotype. In this case, only one allele (usually the wild type) at the single locus is expressed in a doseage dependent manner, which results in an intermediate phenotype. A cross of two intermediate phenotypes (= monohybrid heterozygotes) will result in the reappearance of both parent phenotypes and the intermediate phenotype. There is a 1:2:1 phenotype ratio instead of the 3:1 phenotype ratio found when one allele is dominant and the other is recessive. This lets an organism's genotype be diagnosed from its phenotype without time-consuming breeding tests.The classic example of this is the color of carnations. R R' R RR RR' R' RR' R'R' R is the allele for red pigment. R' is the allele for no pigment.Thus, RR offspring make a lot of red pigment and appear red. R'R' offspring make no red pigment and appear white. Both RR' and R'R offspring make some pigment and therefore appear pink.Another readily visible example of incomplete dominance is the color modifier Merle in dogs.