A. Polygenic inheritance or quantitative inheritance is a type of inheritance of a single phenotypic trait (observable character) is controlled by two or more different genes.
Human skin colour is a good example of polygenic inheritance. Assume that three "dominant" genes (A, B and C) control dark pigmentation because more melanin is produced. The "recessive" alleles of these three genes (a, b, and c) control light pigmentation because lower amounts of melanin are produced. A genotype with all "dominant" genes (AABBCC) has the maximum amount of melanin and very dark skin. A genotype with all "recessive" genes (aabbcc) has the lowest amount of melanin and very light skin. A genotype with genes (AaBbCc) has a medium amount of melanin and an intermediate skin colour.
Three or more alternative forms of a gene that can occupy the same locus all called multiple alleles. However, only two of the alleles can be present in a single organism. For example, the ABO system of blood groups is controlled by a gene (I). It has three alleles (IA, IB, and i). Only two of the three alleles are present in an individual. This phenomenon is called multiple allelism.
B. Pleiotropy is a phenomenon in which one gene, usually mutant allele influences two or more unrelated observable characters (phenotypic traits). The human genetic disease, phenylketonuria (PKU) is a good example of pleiotropy. This disease arises when a single recessive mutant allele is inherited. This allele inactivates the enzyme parahydroxylase which converts phenylalanine to tyrosine. This results in excessive amounts of phenylalanine, a deficiency of tyrosine, and an excess of phenylpyruvic acid which is an alternate degradation product of phenylalanine. These effects can result in mental retardation and also abnormally light hair and skin colour. Such a combination of phenotypic effects of a genetic defect is called a syndrome.
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