Consider the skin pigmentation of biracial individuals, such as Halle Berry, Lenny Kravits, Mariah Carey, or "The Rock." Can these phenotypes (skin colors) be explained by the same Mendelian model?

Skin pigmentation is due to the production of melanin pigment. It is a polygenic trait i.e. it is controlled by many genes. All the alleles of these genes have additive effect which means that each dominant allele contributes a little to the trait. As the number of dominant alleles increase, more contribution occurs and more pigment is produced. Hence, there is a wide range of phenotype and it is present in a continuous spectrum rather than discrete units. Whereas in Mendelian model, a dominant trait always masks the effect of recessive trait and it does not matter if there is one or more than one dominant allele. Hence, according to Mendelian model AA and Aa will have same phenotype but according to additive allele concept they will have different phenotype.

Biracial individuals have different combinations of the skin pigmentation alleles so their skin colors can vary from each other and from their parents' too. Even if they have a mixed skin color the degree of mixing is not same in every individual. Unlike Mendelian trait it is difficult to predict their skin color because so many genes with additive effect are involved.

Hence, skin color can not be explained by Mendelian model.

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