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Mendel's peas were tall or dwarf, purple or white. People are not simply tall or short; height spreads across a range. Polygenic inheritance explains that range, and pleiotropy explains how one faulty gene can show up in several places at once. NEET asks both, often as a ratio or a match-the-term question. This post covers the ideas, a ratio method that never needs a 64-box Punnett square, and the common slips.
What does the syllabus include?
Unit 7 of the NEET (UG) 2026 syllabus lists, under deviations from Mendelism, "Pleiotropy; Elementary idea of polygenic inheritance". The source is sections 4.4 and 4.5 of Principles of Inheritance and Variation in NCERT Class 12 Biology (2026-27 reprint). Check the syllabus for your exam year.
What is polygenic inheritance?
NCERT describes polygenic traits as those controlled by three or more genes, spread across a gradient, and influenced by the environment as well. The effect of each allele is additive: the phenotype depends on how many "adding" alleles you carry, not which gene they sit on.
Its example is human skin colour, assuming three genes A, B and C. The dominant alleles A, B and C add darker colour; a, b and c add lighter. AABBCC is darkest, aabbcc is lightest, and any genotype with three dominant and three recessive alleles is intermediate.
Is height really polygenic?
Yes, on a large scale. A 2022 genome-wide study in Nature by Yengo and colleagues, using data from 5.4 million people, found 12,111 independent DNA variants significantly linked to height. Together they accounted for nearly all of the height variation that common variants are expected to explain. NCERT's three-gene model is a simplification of exactly this kind of trait.
Worked example: the trihybrid ratio
Two parents are both AaBbCc. How many phenotype classes do the offspring show, and in what ratio?
Each parent passes on three alleles, one from each gene, so a child carries six alleles in total from these genes.
Each of the six is dominant with probability 1/2, independently. So the number of dominant alleles follows the binomial pattern for six trials.
The counts of 0, 1, 2, 3, 4, 5 and 6 dominant alleles are in the ratio of the binomial coefficients C(6, k): 1 : 6 : 15 : 20 : 15 : 6 : 1, out of 64.
That gives 7 phenotype classes. The intermediate class, with exactly three dominant alleles, is 20/64 = 5/16 of the offspring. Each extreme is 1/64.
General rule: with n genes and both parents heterozygous for all of them, there are 2n + 1 classes, and each extreme appears in (1/4)ⁿ of the offspring. With two genes, AaBb × AaBb gives 5 classes in the ratio 1 : 4 : 6 : 4 : 1.
To see why the distribution looks bell-shaped, put in sample numbers. Suppose a plant's height starts at 20 cm with no dominant alleles and each dominant allele adds 5 cm (made-up values for practice). Heights then run from 20 cm to 50 cm, the F1 (AaBbCc) is 35 cm, and most F2 plants sit near 35 cm. Our binomial theorem post explains where these coefficients come from.
What is pleiotropy?
Pleiotropy is when one gene affects several phenotypes. NCERT explains that this usually happens because the gene acts on a metabolic pathway that feeds into different traits.
Its example is phenylketonuria (PKU), caused by a mutation in the gene for the enzyme phenylalanine hydroxylase. NCERT lists the effects as intellectual disability and reduced hair and skin pigmentation. The US National Library of Medicine's MedlinePlus page on PKU adds that phenylalanine builds up in the blood, that children with classic PKU tend to have lighter skin and hair than unaffected family members, and that PKU can often be managed with a diet low in phenylalanine. One gene, several effects.
How do the two ideas compare?
Polygenic: many genes, one trait, additive effects, continuous variation. Example: skin colour.
Pleiotropy: one gene, many traits. Example: PKU.
Multiple alleles, often confused with both: one gene with more than two alleles in a population. Example: ABO blood groups.
Which mistakes cost marks?
Using 9:3:3:1 for a polygenic trait. That is two genes with complete dominance and separate traits, not additive effects.
Counting genotypes instead of dominant alleles, which gives 27 classes instead of 7.
Calling ABO blood groups polygenic. They show multiple alleles of one gene.
Forgetting the environment. NCERT says polygenic inheritance also takes environment into account.
Describing PKU as several genes. It is a single-gene mutation with several effects.
For Punnett-square practice on simpler crosses, work through our guide to Mendelian genetics problems.
If the ratio logic still feels like guesswork, Fermi's AI tutor can work out what you already know and build a plan for the session, so you start from the step that is missing.
A practice routine
Write out Pascal's triangle up to the row 1 6 15 20 15 6 1 from memory.
Solve one two-gene and one three-gene polygenic cross each session, giving classes, ratio and the fraction in one named class.
Make a three-column list: polygenic, pleiotropy, multiple alleles, with one NCERT example each.
Explain PKU aloud in three sentences: gene, enzyme, effects.
Start now: for AaBb × AaBb, find the fraction of offspring with exactly one dominant allele. (Answer: 4/16 = 1/4.)
