Botany · Ch 6 — Principles of Inheritance and Variation
Polygenic Inheritance
Polygenic Inheritance
Mendel’s work focused on traits that come in two clear-cut, opposite forms — like purple versus white flowers. But most features in nature are not that simple. Look around: people are not just “tall” or “short” as two separate groups. Instead, height ranges continuously from very short to very tall, with every possible intermediate value in between. Such traits are called polygenic traits.
A polygenic trait is controlled by three or more genes, not just one. Because many genes contribute, the trait shows a continuous range of phenotypes rather than a few discrete classes. Importantly, polygenic inheritance also considers the influence of the environment — the final phenotype is shaped by both genes and external factors.
The word “polygenic” literally means “many genes.” Each gene involved adds a small, cumulative effect.
How polygenic inheritance works: the additive model
In a polygenic trait, each dominant allele contributes a fixed amount to the phenotype, and each recessive allele contributes a smaller (or zero) amount. The overall phenotype is the sum of the contributions of all alleles present. This is called additive inheritance.
Classic example: human skin colour
The NCERT textbook uses human skin colour as the key example. It assumes that three genes — A, B, and C — control skin colour.
- The dominant forms (A, B, C) each contribute to dark skin.
- The recessive forms (a, b, c) each contribute to light skin.
The darkest possible skin colour comes from the genotype AABBCC — all six alleles are dominant. The lightest possible skin colour comes from aabbcc — all six alleles are recessive.
An individual with three dominant and three recessive alleles (for example, AaBbCc) will have an intermediate skin colour, exactly midway between the two extremes.
The number of dominant alleles in the genotype directly determines how dark or light the skin will be. More dominant alleles = darker skin; fewer dominant alleles = lighter skin. This is a purely additive system.
Key features of polygenic inheritance (from the textbook)
- Controlled by three or more genes.
- Phenotypes show a continuous range (a gradient), not discrete alternatives.
- The effect of each allele is additive — the phenotype reflects the total number of dominant alleles.
- Environment also influences the final expression of the trait.
Examples mentioned in the textbook
| Trait | Description |
|-------|-------------| …