Pleiotropy

What is pleiotropy?

In plain words: earlier examples followed one trait at a time. Sometimes a single gene affects several features, in most cases because it acts on a metabolic pathway that feeds into different phenotypes (NCERT page 69).

Pleiotropic genes

• So far the chapter has looked at the effect of a gene on a single phenotype or trait. In pleiotropy, a single gene has multiple phenotypic expressions (page 69).
• In most cases this happens because the gene acts on a metabolic pathway, and that pathway contributes to different phenotypes (page 69).
• NCERT gives a plant example earlier in the chapter. “Occasionally, a single gene product may produce more than one effect”: the pea gene for starch synthesis (alleles B and b) sets both the size of the starch grains and the shape of the seed, round or wrinkled (NCERT Section 4.2, page 62).
• Pleiotropy is the reverse of polygenic inheritance, where a trait is generally controlled by three or more genes (NCERT Section 4.4, page 69; beyond the NCERT text, NHGRI says two or more).

Phenylketonuria as a pleiotropic disorder

• NCERT: the disease is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation) (page 69); the affected individual lacks the enzyme that converts phenylalanine into tyrosine (page 75). Give NCERT’s account in an NCERT-based answer.
• Beyond the NCERT text: the gene is PAH, both copies carry disease-causing variants, and enzyme activity may be very low rather than absent (MedlinePlus Genetics, Phenylketonuria).
• NCERT: the mutation in this single gene manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation (page 69); phenylalanine accumulates and is converted into phenylpyruvic acid and other derivatives, whose accumulation in the brain results in mental retardation (page 75). These are multiple consequences of one gene: pleiotropy.
• Beyond the NCERT text: “mental retardation” is an older term. MedlinePlus Genetics (US National Library of Medicine, public domain) says: “If PKU is not treated, phenylalanine can build up to harmful levels in the body, causing intellectual disability and other serious health problems.” Where an answer must follow NCERT’s wording, use NCERT’s phrase; otherwise use the current term.
• PKU is an autosomal recessive inborn error of metabolism (NCERT page 75). Beyond the NCERT text: MedlinePlus Genetics notes that “PKU can often be managed by following a diet that is low in phenylalanine”, so early detection and treatment can prevent major neurological harm; disability is not an inevitable consequence of the genotype.

NCERT wording and current interpretation: NCERT uses the older term “mental retardation” and attributes brain effects to accumulated derivatives (page 75). In an NCERT-based answer, give NCERT’s account. Beyond the NCERT text: use intellectual disability in explanatory prose; MedlinePlus Genetics describes phenylalanine itself building up to harmful levels (quoted above). NCERT’s “poor absorption by kidney” refers to the kidney’s poor reabsorption of the derivatives (our reading).

Solved examples

Common mistakes to avoid

Quick revision summary

• A pleiotropic gene is a single gene with multiple phenotypic expressions.
• In most cases it acts through a metabolic pathway that contributes to several phenotypes.
• PKU: a mutation in the single gene for phenyl alanine hydroxylase shows itself as mental retardation (NCERT’s term; now intellectual disability) and a reduction in hair and skin pigmentation, illustrating pleiotropy (NCERT page 69).
• PKU is autosomal recessive: the affected individual lacks the enzyme that converts phenylalanine into tyrosine; phenylalanine accumulates and is converted into phenylpyruvic acid and other derivatives, whose accumulation in the brain results in mental retardation; they are also excreted in urine (NCERT page 75).
• In pea, the starch-synthesis gene affects both starch grain size and seed shape.

Questions students ask

What is the difference between pleiotropy and epistasis?

Pleiotropy is one gene affecting several traits, while epistasis, a term beyond the NCERT text, is one gene changing how another gene is expressed: NHGRI’s glossary describes the expression of one gene being “modified (e.g., masked, inhibited or suppressed) by the expression of one or more other genes”. Pleiotropy therefore starts from a single gene and its many effects, as in phenylketonuria (NCERT page 69), while epistasis needs at least two genes acting on one trait. NHGRI’s example, also beyond the NCERT text, is coat colour in Labrador retrievers: one gene sets black or chocolate pigment, and a recessive form of a second gene keeps that pigment out of the fur, so the dog is yellow.

What is the difference between pleiotropy and multiple alleles?

Pleiotropy is one gene producing several phenotypic effects, while multiple alleles means one gene existing in more than two allelic forms: NCERT’s examples are phenylketonuria for the first (page 69) and the ABO gene I, with its three alleles I^A, I^B and i, for the second (pages 61–62). Since a person carries only two alleles of a gene, multiple alleles “can be found only when population studies are made” (page 62), whereas pleiotropy is visible in one person, such as someone with phenylketonuria, who has its several effects together.

Is sickle-cell anaemia an example of pleiotropy?

Sickle-cell anaemia is an example of pleiotropy by our reading of NCERT’s definition, a single gene with multiple phenotypic expressions (page 69), though NCERT itself names only phenylketonuria. In Hb^SHb^S people one point mutation in the beta globin gene alters haemoglobin (NCERT pages 74–75), and, beyond the NCERT text, the sickling of red blood cells this can cause leads to anaemia, episodes of pain, repeated infections and damage to the brain, kidneys, lungs or spleen (MedlinePlus Genetics, 2024). A NEET question asking for NCERT’s example of pleiotropy still wants phenylketonuria.

Is pleiotropy a deviation from Mendel’s laws?

Pleiotropy is a deviation from Mendelism, not a breach of Mendel’s laws: each of Mendel’s seven traits was followed as the effect of one gene, while a pleiotropic gene has several effects, and the NEET (UG) 2026 syllabus lists pleiotropy among the “Deviations from Mendelism”. Its alleles still segregate as Mendel’s law requires, a gamete receiving “only one of the two factors” (NCERT page 59). In phenylketonuria, NCERT’s example, all the effects come from one gene (page 69), so they are inherited together as one Mendelian unit and no cross can separate them.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Name the enzyme whose gene is mutated in phenylketonuria, and the reaction it carries out.

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   Phenylalanine hydroxylase (NCERT page 69); it converts the amino acid phenylalanine into tyrosine (NCERT page 75).

2. Q2. What is the underlying mechanism of pleiotropy in most cases?

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   The effect of a gene on metabolic pathways that contribute towards different phenotypes (NCERT page 69).

3. Q3. Why are phenylpyruvic acid and the other derivatives excreted in urine in phenylketonuria?

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   The derivatives are excreted through urine because of their poor absorption by kidney (NCERT page 75).