Medicneet Logo

Polygenic Inheritance (Human Skin Colour) Explained

Biology · Principles of Inheritance and Variation · NEET

Polygenic inheritance is when one single trait is controlled by three or more genes together, not just one. Each dominant allele adds a little effect (additive effect), and the environment also matters, so you get a smooth range of results like human skin colour or height. Memory hook: POLY = many genes push ONE trait, and each dark allele "adds one shade darker."
Polygenic Inheritance: Human Skin Colour (3 genes A, B, C)More dominant (dark) alleles = darker skin. Effects are additive.aabbccAaBbccAaBbCcAABbCcAABBCC0 dark2 dark3 dark5 dark6 darkLightestDarkestContinuous gradient of phenotypes + environment effect
In NCERT's model, three genes (A, B, C) control skin colour. Each dominant allele adds one step of darkness, so the number of dominant alleles decides the shade. This additive effect (0 to 6 dark alleles) plus environment gives a smooth range from lightest (aabbcc) to darkest (AABBCC).

Your doubts, answered

What exactly does 'polygenic' mean in simple words?

'Poly' means many and 'genic' means genes. So polygenic means one single character (like skin colour) is controlled by MANY genes working together, usually three or more. This is the opposite of what Mendel studied, where one gene controlled one trait. NCERT gives human skin colour and human height as the classic examples.

Why do we get a whole RANGE of skin colours instead of just two types?

Because the effect of each allele is ADDITIVE. NCERT assumes 3 genes A, B, C control skin colour. Each dominant allele (A, B or C) adds a bit of darkness. So AABBCC (6 dominant) = darkest, aabbcc (0 dominant) = lightest, and in between you get many shades depending on how many dominant alleles you have. More dominant alleles = darker skin. That is why skin colour is a smooth gradient, not two boxes.

What is the 'additive effect' of alleles?

Additive effect means each dominant allele adds an equal small amount to the trait, and you simply add them up. It is like counting: 6 dominant alleles give the darkest skin, 3 give medium, 0 give the lightest. No single allele hides another one (no dominance covering up). The final phenotype = sum of all contributing alleles. This is why NCERT says 'the effect of each allele is additive.'

Is polygenic inheritance Mendelian or non-Mendelian?

It is NON-Mendelian. NEET 2025 asked this directly. Mendel studied traits with distinct alternate forms (tall OR dwarf, purple OR white). Polygenic traits instead show a continuous gradient controlled by many genes plus environment, so they do not follow simple Mendelian ratios. Remember: polygenic = non-Mendelian = continuous variation.

Does the environment affect polygenic traits?

Yes. NCERT clearly states polygenic inheritance 'also takes into account the influence of environment.' For skin colour, sun exposure can make skin darker even with the same genes. So a polygenic phenotype = genes (additive) + environment. This is one big difference from simple Mendelian traits, where environment is usually ignored.

How is polygenic inheritance different from multiple alleles (like blood group)?

Do not mix these up. Multiple alleles means ONE gene has more than two allele forms (blood group gene I has IA, IB, i) but it is still ONE gene controlling ONE trait. Polygenic means MANY different genes (three or more) control ONE trait. NEET 2018 confirmed blood group shows multiple alleles, NOT polygenic inheritance. Blood group = one gene, many alleles; skin colour = many genes, one trait.

What is the difference between polygenic inheritance and pleiotropy?

They are opposites. Polygenic = MANY genes control ONE trait (skin colour). Pleiotropy = ONE gene controls MANY traits (phenylketonuria affects many features). NEET loves to swap these in matching questions. Memory line: polygenic is 'many-to-one', pleiotropy is 'one-to-many'.

⚠️ The NEET trap
✗ Human skin colour shows multiple alleles / is a Mendelian trait, like blood groups.
✓ Human skin colour is polygenic (three or more genes, additive effects, plus environment) and is a NON-Mendelian, continuous trait. Multiple alleles applies to blood groups, not skin colour.
🧠 In matching questions NTA pairs 'Polygenic inheritance -> Human skin colour'. Do not confuse polygenic (many genes -> one trait) with multiple alleles (one gene -> many alleles, blood group) or pleiotropy (one gene -> many traits, phenylketonuria).

Real NEET questions

2025

What is the pattern of inheritance for polygenic trait?

A · Autosomal dominant pattern
B · X-linked recessive inheritance pattern
C · Mendelian inheritance pattern
D · Non-mendelian inheritance pattern ✓
Solution: Polygenic traits are controlled by three or more genes whose effects are additive and are also influenced by the environment, giving a continuous range of phenotypes instead of distinct classes. This is unlike the distinct alternate forms Mendel studied, so polygenic inheritance is a NON-Mendelian pattern. (NCERT Class XII, Ch 4, p.69)
2026

Match List I with List II : List-I: A. Incomplete dominance B. Co-dominance C. Pleiotropy D. Polygenic inheritance List-II: I. Human skin colour II. Inheritance of flower colour in Antirrhinum sp. III. Phenylketonuria disease in humans IV. ABO blood groups

A · A-II, B-IV, C-III, D-I ✓
B · A-I, B-III, C-II, D-IV
C · A-I, B-IV, C-III, D-II
D · A-II, B-I, C-III, D-IV
Solution: Incomplete dominance = flower colour in Antirrhinum/snapdragon (A-II). Co-dominance = ABO blood groups where IA and IB both express (B-IV). Pleiotropy = phenylketonuria, one gene many effects (C-III). Polygenic inheritance = human skin colour, many genes one trait (D-I). So the answer is A-II, B-IV, C-III, D-I.
2018

Which of the following characteristics represent 'Inheritance of blood groups' in humans? a. Dominance b. Co-dominance c. Multiple allele d. Incomplete dominance e. Polygenic inheritance

A · b, d and e
B · a, b and c ✓
C · b, c and e
D · a, c and e
Solution: ABO blood group shows dominance (IA, IB dominant over i), co-dominance (IA and IB together both express), and multiple alleles (gene I has IA, IB, i). It does NOT show incomplete dominance or polygenic inheritance. So the correct set is a, b and c. This is a good trap: blood group is multiple alleles, NOT polygenic.

Solved Principles of Inheritance and Variation NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

See all 65 Principles of Inheritance and Variation NEET PYQs ›
Next concept: What is Pleiotropy? One Gene, Many Traits (NEET) →Keep learning — 2 minFeeling ready? Solve the Principles of Inheritance and Variation NEET PYQs ›Or practice on your phone — get the free MedicNEET app ›

Frequently asked

How many genes control human skin colour according to NCERT?

NCERT uses a model of three genes (A, B, C) to explain it, but polygenic traits are controlled by 'three or more' genes in general. For NEET, remember 'three or more genes' as the key phrase.

Give two examples of polygenic inheritance from NCERT.

Human skin colour and human height (a whole range of heights, not just tall or short). Both show continuous variation controlled by many genes plus environment.

What genotype gives the darkest and the lightest skin?

With NCERT's 3-gene model, AABBCC (all six dominant alleles) gives the darkest skin and aabbcc (all six recessive alleles) gives the lightest. A mix like three dominant and three recessive gives an intermediate shade.

Why is polygenic inheritance called continuous variation?

Because adding one more dominant allele changes the trait by only a small step, so the phenotypes form a smooth gradient (many in-between values) instead of a few clear-cut classes. Environment adds even more smoothing.

Is polygenic inheritance important for NEET?

Yes. It appears in matching questions (paired with human skin colour) and in direct questions like the 2025 'non-Mendelian pattern' question. You must also separate it clearly from pleiotropy and multiple alleles, which are common traps.

Related concepts:
What is pleiotropy (one gene, many traits)Difference between pleiotropy and polygenic inheritanceMultiple alleles and ABO blood group inheritanceWhat is incomplete dominance (Snapdragon pink flower)Difference between genotype and phenotype
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Principles of Inheritance and Variation PYQs