(a) Explain polygenic inheritance and multiple allelism with the help of suitable examples.

(b) "Phenylketonuria is a good example that explains Pleiotropy." Justify. [5]

(a) Polygenic Inheritance : Polygenic inheritance ocurs when traits are controlled by three or more genes. The phenotype reflects the contribution of each allele i.e., effect of each allele is additive. Example: Human skin colour is controlled by three genes such as A, B and C. [1]

Multiple Allelism : In multiple allelism, more than two alleles govern the same character or phenotype. Example: Human blood group-ABO system is controlled by three different alleles-$I^A$, $I^B$, $I^O$. [1]

(b) In pleiotropy, a single gene can exhibit multiple phenotypic expressions, e.g., phenylketonuria. In phenylketonuria, the affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. As a result, phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives which results in mental retardation and reduction in hair and skin pigmentation. [3]