Q.Write notes on Polygenic inheritance (with 2 to 3 important points). OR Write notes on Linkage (with 2 to 3 important points).

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 …

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.

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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.

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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 (sin …

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.

Working

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. …

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. Th …

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. ✓ …