Q.Define pleiotropy. Explain, with the example of phenylketonuria (PKU) in humans, how a mutation in a single gene can affect several apparently unrelated phenotypic characters.

Pleiotropy is the phenomenon where a single gene influences multiple, apparently unrelated phenotypic traits.

• Multiple allele refers to a single gene having more than two allelic forms in a population (e.g. ABO blood group), not one gene controlling many traits.

• Epistasis is an interaction between two different genes, where one gene masks or modifies the expression of another.

• Pleiotropy is when one single gene affects several, seemingly unconnected phenotypic characters simultaneously — e.g. the gene for phenylketonuria (PKU) in humans affects mental ability, hair/skin pigmentation, and other traits together.

• Incomplete dominance describes a heterozygote showing an intermediate phenotype between the two homozygotes (e.g. Mirabilis jalapa flower colour).

Phenylketonuria is the standard example of pleiotropy.

Pleiotropy is the phenomenon where one gene affects more than one phenotypic character simultaneously.

Normally we think of one gene controlling one trait, but many genes actually influence several traits at once because their protein product participates in more than one metabolic pathway or developmental process. This phenomenon is called pleiotropy.

• Example: in humans, the gene for phenylketonuria (PKU) causes not only mental retardation but also reduced hair and skin pigmentation, because the same faulty enzyme affects multiple biochemical pathways.

• This is different from the other options: codominance is when both alleles of a gene express themselves fully and independently in the heterozygote (e.g., AB blood group); incomplete dominance is when the heterozygote shows an intermediate phenotype (e.g., pink flowers in Mirabilis jalapa); polygenic inheritance is the reverse situation - multiple genes together controlling a single trait (e.g., human skin colour).

Pleiotropy = one gene, multiple phenotypic effects.

Normally we think of one gene controlling one trait, but many genes affect several, apparently unrelated, characters at the same time — this is called pleiotropy. It happens because the gene's product (often an enzyme early in a metabolic pathway) is involved in more than one biochemical process, so a mutation in it shows up as multiple phenotypic changes.

Example: The gene responsible for phenylketonuria (PKU) in humans, besides causing mental retardation, also affects hair and skin pigmentation (a phenylalanine-hydroxylase deficiency disrupts melanin production as well).