Q.What is polygenic inheritance? Explain briefly, using the example of the inheritance of skin colour in humans, how three gene pairs acting together can produce a continuous range of phenotypes.

Human skin colour, controlled additively by multiple genes and showing continuous variation, is the example of polygenic inheritance among the four options.

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Polygenic inheritance refers to traits controlled by two or more genes (loci) acting together in an additive fashion, so that the phenotype shows continuous (quantitative) variation across a range, rather than falling into a small number of sharply distinct classes, and is often also influenced by environmental factors. Checking each option: pod shape in garden pea (inflated vs constricted) is one of Mendel's seven classic traits, governed by a single gene showing simple dominant-recessive inheritance -- not polygenic. Flower colour in Mirabilis jalapa (red x white giving pink) is the classic textbook example of incomplete dominance, still controlled by a single gene with two alleles that blend in the heterozygote -- again not polygenic. Production of male honeybees is explained by haplodiploidy: unfertilised (haploid) eggs develop into males (drones), while fertilised (diploid) eggs develop into females (workers/queens) -- this is a chromosomal sex-determination mechanism, not polygenic inheritance. Skin colour in humans, by contrast, is controlled by multiple genes, each contributing a small additive amount of melanin production, so that the observed range of human skin tones forms a continuous spectrum rather than a few discrete categories -- this is the textbook example of polygenic inheritance among the four.

Polygenic inheritance = a trait controlled by many genes with additive effects (skin colour, wheat grain colour, height). ABO blood group is controlled by three alleles of ONE gene (multiple allelism), so it is not polygenic.

Polygenic (quantitative) inheritance involves several genes each adding a small, cumulative effect, producing continuous variation. Examples: human skin colour, grain (kernel) colour in wheat, and human height — all show a gradient of phenotypes.

The ABO blood group, in contrast, is governed by three alleles (Iᴬ, Iᴮ, i) of a single gene — a case of multiple allelism (with co-dominance between Iᴬ and Iᴮ), not polygenic inheritance. Therefore ABO blood group is the odd one out.

A frequently asked NCERT/CBSE Class 12 Biology genetics discrimination question.

Skin colour in humans is determined by several genes acting together — polygenic (multiple-gene) inheritance.

Skin colour is a classic example of polygenic (quantitative) inheritance, controlled by multiple genes (chiefly those governing melanin production). The combined effect of several genes produces a continuous range of skin shades from very light to very dark, rather than distinct categories.

Because it is governed by many genes together, the determining factor is multiple genes (polygenes); it is a quantitative, not a qualitative, character.

Skin colour is polygenic because several genes, each with a small cumulative (additive) effect, jointly determine the total amount of melanin pigment, producing a continuous range of shades.

In polygenic inheritance, a phenotype is controlled by three or more genes, and the effect of each individual gene's alleles is small but additive — the more "effective" (pigment-producing) alleles an individual has, the more extreme the phenotype. Human skin colour is a classic example: it is governed by at least three genes (each with a dark and a light allele), and the cumulative dosage of the dark alleles across all these genes determines how much melanin is produced. Because there are several genes each contributing incrementally, the population shows a continuous gradation of skin colours (from very fair to very dark) rather than a few sharply distinct classes, as would be seen with a simple monogenic (single-gene) trait. Environmental factors (e.g. sun exposure) can further modulate melanin production, adding to the continuous variation.

Human skin colour, controlled additively by several genes to give a continuous range of shades, is the classic example of polygenic inheritance among the options given.

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Polygenic inheritance occurs when a single phenotypic trait is controlled not by one gene but by several genes acting together, each contributing a small, additive effect, so that the trait shows continuous (quantitative) variation across a whole range of intermediate phenotypes rather than a few discrete classes -- the population distribution for such a trait typically approximates a bell (normal) curve. Human skin colour is the standard textbook illustration: multiple gene pairs (classically estimated as 3 or 4) each add or subtract pigment (melanin) in an additive fashion, producing the wide, continuous spectrum of human skin shades seen across populations.

By contrast:

• Pod shape in garden pea (inflated dominant over constricted) is a simple monohybrid (single-gene) Mendelian trait, not polygenic.
• Flower colour in Mirabilis jalapa (red x white giving pink F1) is a single-gene trait showing incomplete dominance, again not polygenic.
• Production of a male honey bee (drone) from an unfertilised, haploid egg (arrhenotoky/haplodiploidy) is a chromosomal mechanism of sex determination, unrelated to polygenic inheritance.

So, of the four, skin colour in humans is the genuine example of polygenic inheritance.

Skin colour results from the additive, cumulative effect of several genes, so it shows continuous (quantitative) variation — this is polygenic inheritance.

Unlike traits controlled by a single gene, human skin colour is governed by at least three genes, each with an additive (non-dominant, cumulative) effect. The more 'dark alleles' an individual inherits, the darker the skin tone, and the fewer, the lighter — producing a continuous gradation of colour in the population rather than distinct classes. This cumulative, additive action of multiple genes on a single phenotype is called polygenic inheritance, and it is also strongly influenced by environmental factors like sun exposure.

The key idea is that skin colour in humans is a polygenic trait where each dominant allele adds pigment. The darkest skin requires all dominant alleles, so Assertion (A) is true; Reason (R) correctly explains that phenotype reflects the contribution of each allele, making both true and (R) the correct explanation.

Polygenic inheritance = a trait controlled by many genes; human skin colour is the standard example — the answer is (a).

Polygenic inheritance is the control of a single character by two or more genes, where each contributing (additive) allele adds a small quantitative effect, producing a continuous range of phenotypes.

• (a) Human skin colour — governed by multiple genes (e.g. three or more) whose additive alleles determine the amount of melanin, giving a whole gradient from very dark to very fair. This is the textbook example of polygenic inheritance. ✓

• (b) Sickle-cell anaemia — single-gene point mutation; (c) ABO blood group — multiple alleles of one gene; (d) cystic fibrosis — single-gene disorder. None of these is polygenic.