, Pleiotropy is the phenomenon where a single gene exhibits multiple phenotypic expressions. A classic example given is the disease phenylketonuria, where a mutation in a single gene leads to mental retardation and reduction in hair and skin pigmentation . In contrast, Polygenic inheritance involves traits controlled by three or more genes (e.g., human skin colour) . Co-dominance occurs when the F1 generation resembles both parents (e.g., ABO blood group AB) . Multiple alleles refers to the presence of more than two alleles governing the same character (e.g., ABO blood grouping) .