Study Guide

Polygenic Inheritance

CIE A-Level BiologyΒ· 25 min read

1. What is Polygenic Inheritance?β˜…β˜…β˜…β˜†β˜†β± 8 min

πŸ“˜ Definition

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.

πŸ“ 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 Γ— AaBbCc)?

  1. 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
    1. 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. 3
    P(0)=(30)(12)3=18P(1)=(31)(12)3=38P(2)=(32)(12)3=38P(3)=(33)(12)3=18\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. 4
    1. Sum the probabilities of all allele combinations that add to 3 dominant alleles:
  5. 5
    P(3)=(P(0)Γ—P(3))+(P(1)Γ—P(2))+(P(2)Γ—P(1))+(P(3)Γ—P(0))P(3) = (P(0) \times P(3)) + (P(1) \times P(2)) + (P(2) \times P(1)) + (P(3) \times P(0))
  6. 6
    1. Calculate the final probability:
  7. 7
    P(3)=(18Γ—18)+(38Γ—38)+(38Γ—38)+(18Γ—18)=2064=516P(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. 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.

2. Polygenic Inheritance and Continuous Variationβ˜…β˜…β˜†β˜†β˜†β± 7 min

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.

πŸ“˜ Definition

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.

πŸ“ 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
    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
    1. 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
    1. 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
    1. Environmental variation adds further spread to the distribution, smoothing out any small discrete steps between genotypes to produce a fully continuous range.

3. Comparing Polygenic and Monogenic Inheritanceβ˜…β˜…β˜…β˜†β˜†β± 6 min

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

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Examples in humans

ABO blood group, cystic fibrosis

Height, skin colour, body mass

βœ“ Quick check

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

    Reveal answer
    Human adult height β€”

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

4. Common Pitfalls

Wrong move:

Confusing polygenic inheritance with multiple alleles

Why:

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 move:

Remember: Multiple alleles = one gene, many alleles; polygenic = many genes, one trait.

Wrong move:

Claiming polygenic inheritance produces discontinuous variation

Why:

Discontinuous variation is caused by monogenic inheritance, while additive effects of multiple genes produce a continuous range of phenotypes.

Correct move:

Always link polygenic inheritance to continuous variation and monogenic inheritance to discontinuous variation in exam answers.

Wrong move:

Ignoring environmental influence on polygenic traits

Why:

Nearly all polygenic traits are multifactorial, and exam questions expect you to mention the interaction between genes and environment.

Correct move:

Always include a reference to environmental effects when explaining the distribution of polygenic traits.

Wrong move:

Miscounting dominant alleles when calculating polygenic phenotype probabilities

Why:

Most exam problems use additive allele inheritance, so wrong counts lead to incorrect proportion calculations.

Correct move:

List all possible combinations of dominant alleles before calculating probability, and double-check your sum.

5. Quick Reference 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

When this came up on past exams

AI-estimated based on syllabus patterns β€” cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2022 Β· 22

    Compare polygenic and monogenic inheritance

  • 2023 Β· 12

    Explain polygenic cause of continuous variation

Going deeper

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.