Q.When many phenotypic traits are controlled by only one gene, this phenomenon is known as

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. …

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. …

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.

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