# Polygenic Inheritance

> CIE A-Level Biology · Unit 17: Inheritance
> Source: https://www.owlsprep.com/study/cie-9700-u17-polygenic-inheritance/

This sub-topic explains how characteristics controlled by multiple genes produce continuous phenotypic variation, compares polygenic and monogenic inheritance patterns, and prepares you for common exam questions on this topic.

**Prerequisites:** [Monogenic inheritance and Punnett squares](https://www.owlsprep.com/study/cie-9700-u17-monogenic-inheritance/); [Types of phenotypic variation](https://www.owlsprep.com/study/cie-9700-u16-variation-types/)

## Learning objectives

- Distinguish between polygenic and monogenic inheritance
- Explain how polygenic inheritance produces continuous phenotypic variation
- Identify common examples of polygenic traits
- Interpret exam data on polygenic inheritance patterns

## What is Polygenic Inheritance?

**Polygenic Inheritance** — A pattern of inheritance where one phenotypic characteristic is controlled by two or more different genes located at separate loci on different chromosomes. Most polygenic traits are also influenced by environmental factors.

*Example:* Human height, skin colour, and wheat grain colour are all classic examples of polygenic traits.

Unlike monogenic inheritance (one gene controls one trait), polygenic inheritance involves additive effects of multiple alleles. Each dominant allele typically adds a small incremental contribution to the final phenotype, leading to a wide range of possible phenotypic outcomes.

**Worked example:** Wheat grain colour is controlled by three unlinked genes, each with two alleles (A/a, B/b, C/c). Each dominant allele adds one unit of red pigment. What is the probability of producing a grain with 3 units of pigment from a cross between two double heterozygous parents (AaBbCc × AaBbCc)?

1. 1. We need 3 total dominant alleles in the offspring to get 3 units of pigment, since each dominant allele contributes one unit.
2. 2. Calculate the probability of getting n dominant alleles from a single AaBbCc parent, where the probability of passing a dominant allele for any gene is 1/2:
3. $$\begin{aligned} P(0) &= \binom{3}{0}(\frac{1}{2})^3 = \frac{1}{8} \\ P(1) &= \binom{3}{1}(\frac{1}{2})^3 = \frac{3}{8} \\ P(2) &= \binom{3}{2}(\frac{1}{2})^3 = \frac{3}{8} \\ P(3) &= \binom{3}{3}(\frac{1}{2})^3 = \frac{1}{8} \end{aligned}$$
4. 3. Sum the probabilities of all allele combinations that add to 3 dominant alleles:
5. $$P(3) = (P(0) \times P(3)) + (P(1) \times P(2)) + (P(2) \times P(1)) + (P(3) \times P(0))$$
6. 4. Calculate the final probability:
7. $$P(3) = (\frac{1}{8} \times \frac{1}{8}) + (\frac{3}{8} \times \frac{3}{8}) + (\frac{3}{8} \times \frac{3}{8}) + (\frac{1}{8} \times \frac{1}{8}) = \frac{20}{64} = \frac{5}{16}$$
8. The final probability is 5/16.

> **Exam tip:** When calculating phenotype proportions for additive polygenic traits, always count the number of dominant alleles per genotype before working out probability.

## Polygenic Inheritance and Continuous Variation

Polygenic inheritance is the primary genetic cause of continuous variation, where phenotypes do not fall into discrete groups, instead forming a smooth gradient from one extreme to another. This contrasts sharply with discontinuous variation (typical of monogenic inheritance), where phenotypes fall into clear, separate categories.

**Continuous Variation** — Variation in a phenotypic trait where there is a complete range of values between two extremes, with no clear separation between different phenotype groups.

*Example:* Human adult height, which ranges from very short to very tall with all intermediate heights possible.

> **info**
>
> Almost all polygenic traits are multifactorial, meaning they are influenced by both genetics and environmental factors. For example, human height is genetically determined by multiple genes, but childhood nutrition has a significant effect on final adult height.

**Worked example:** A study measured height in 1000 adult human females. Describe the expected distribution of heights and explain why this shape occurs in polygenic traits.

1. 1. The expected distribution is a symmetric, bell-shaped normal distribution. Most individuals have heights close to the population mean, with far fewer individuals at the very short or very tall extremes.
2. 2. Because polygenic traits are controlled by multiple genes with small additive effects, most offspring inherit a mix of increasing and decreasing alleles, leading to an intermediate phenotype near the mean.
3. 3. Only a very small number of individuals inherit almost all alleles that increase (or decrease) the trait, leading to the rare extreme phenotypes at either end of the distribution.
4. 4. Environmental variation adds further spread to the distribution, smoothing out any small discrete steps between genotypes to produce a fully continuous range.

## Comparing Polygenic and Monogenic Inheritance

| Feature | Monogenic Inheritance | Polygenic Inheritance |
| --- | --- | --- |
| Number of genes controlling trait | 1 gene at one locus | 2 or more genes at separate loci |
| Effect of individual alleles | Usually large effect per allele | Small additive effect per allele |
| Examples in humans | ABO blood group, cystic fibrosis | Height, skin colour, body mass |

**Exam command terms**

Common command terms for this topic in CIE exams have clear expectations:

- **Distinguish** — State clear, paired differences between two concepts (e.g. polygenic vs monogenic) *(You need at least two distinct differences to get full marks)*

- **Explain** — Give the causal mechanism linking polygenic inheritance to continuous variation *(You must mention multiple genes, additive effects, and environmental influence to get full marks)*

**Check your understanding**

Test your understanding:

1. Which of the following is most likely to be a polygenic trait?

   - ABO blood group in humans
   - Human adult height
   - Sickle cell anaemia
   - Cystic fibrosis

   *Why:* Correct! Height is controlled by multiple genes and shows continuous variation. All other options are monogenic traits with discontinuous variation.

## Common pitfalls

- **Wrong:** Confusing polygenic inheritance with multiple alleles
  - Why it fails: Multiple alleles refers to more than two alleles for a single gene at one locus, while polygenic inheritance refers to multiple separate genes controlling one trait.
  - Correct: Remember: Multiple alleles = one gene, many alleles; polygenic = many genes, one trait.
- **Wrong:** Claiming polygenic inheritance produces discontinuous variation
  - Why it fails: Discontinuous variation is caused by monogenic inheritance, while additive effects of multiple genes produce a continuous range of phenotypes.
  - Correct: Always link polygenic inheritance to continuous variation and monogenic inheritance to discontinuous variation in exam answers.
- **Wrong:** Ignoring environmental influence on polygenic traits
  - Why it fails: Nearly all polygenic traits are multifactorial, and exam questions expect you to mention the interaction between genes and environment.
  - Correct: Always include a reference to environmental effects when explaining the distribution of polygenic traits.
- **Wrong:** Miscounting dominant alleles when calculating polygenic phenotype probabilities
  - Why it fails: Most exam problems use additive allele inheritance, so wrong counts lead to incorrect proportion calculations.
  - Correct: List all possible combinations of dominant alleles before calculating probability, and double-check your sum.

## Cheatsheet

| Term | Key Features | Example |
| --- | --- | --- |
| Polygenic inheritance | Multiple genes control 1 trait, additive allele effects | Human height |
| Monogenic inheritance | Single gene controls 1 trait | ABO blood group |
| Continuous variation | Range of phenotypes, bell-shaped distribution | Human skin colour |
| Discontinuous variation | Distinct, separate phenotype groups | Blood group type |
| Multifactorial trait | Influenced by both genes and environment | Crop grain yield |

## What's next

Polygenic inheritance is a core concept for understanding variation in natural populations, and it underpins all topics related to natural selection and adaptive evolution. Many common human genetic diseases are also polygenic and multifactorial, so this concept is important for questions on genetic health and biotechnology. The continuous variation produced by polygenic inheritance is the raw material for natural selection, so building a solid understanding here will help you with topics on evolution and speciation.

- [Natural Selection](https://www.owlsprep.com/study/cie-9700-u18-natural-selection/)
- [Selection and Evolution](https://www.owlsprep.com/study/cie-9700-u18-overview/)
- [Variation](https://www.owlsprep.com/study/cie-9700-u18-variation/)

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