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D3.2.14—Continuous variation

Continuous variation produces a range of phenotypes, often influenced by many genes and environmental factors in inheritance problems in inheritance problems.

Syllabus
First assessment 2025
Objective
D3.2.14
Level
HL

Exam analysis

Chance of appearing12%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1–3

Common command terms

  • Identify
  • State
  • Outline
  • Distinguish
  • Explain

Recent exam appearances

May 2025Paper1B ["HL"] · TZ14(b)[ 2 ]D3.2.14—Continuous variation
May 2024Paper1 ["HL"] · TZ136[ 1 ]D3.2.14—Continuous variation
May 2022Paper2 ["HL"] · TZ18(c)[ 7 ]D3.2.14—Continuous variation
November 2021Paper2 ["HL"] · TZ06(c)[ 4 ]D3.2.14—Continuous variation
November 2018Paper2 ["HL"] · TZ07(b)[ 5 ]D3.2.14—Continuous variation

Coverage 2013–2025 · Updated 16 Jul 2026

Continuous and Discrete Variation Have Different Data Shapes

Feature Continuous variation Discrete variation
possible values many intermediate measurements separate categories
common cause polygenic effects plus environment often one or few major loci
examples height, body mass, skin pigmentation ABO group, biological sex chromosome category
useful display histogram or box plot bar chart or counts table

Ask whether the variable is measured on a scale or counted in named categories. Height remains continuous even if a researcher later groups measurements into artificial height bands.

Continuous does not mean every imaginable value must occur in one sample, and discrete does not mean the categories are controlled by only one allele.

Many Small Genetic Effects and Environment Produce a Phenotypic Range

In polygenic inheritance, several loci contribute to one characteristic. Different combinations of small allele effects generate many genotypic values rather than a few Mendelian phenotype classes.

many contributing genotypes + nutrition, health, temperature or other environmental effects + genotype-specific responses to those conditions → overlapping phenotypes across a continuous range

Trait Genetic contribution Environmental contribution
height many growth-related loci nutrition, disease and developmental conditions
body mass metabolism and appetite loci diet, activity and health
skin pigmentation several melanin-related loci ultraviolet exposure changes melanin production

Do not confuse multiple genes contributing to one trait with multiple alleles at one gene locus. These are different levels of genetic variation.

Human Inheritance Evidence Runs from Chromosomes to Families to Populations

X and Y contributions establish the usual chromosomal starting point → unequal X-linked gene dosage changes inheritance risk → pedigrees reveal transmission across families → polygenes and environment create continuous population variation → box plots summarize centre and spread

Evidence source Reliable question
X-linked cross which parent supplies the allele to sons or daughters?
pedigree which inheritance hypotheses are contradicted by family relationships?
continuous measurements where is the median and how wide is the middle 50%?
apparent outlier biological extreme, data error or different process?

Match the claim to the evidence scale. One Punnett grid predicts a fertilization probability, a pedigree constrains family genotypes, and a box plot describes a sample distribution; none alone proves a universal biological rule.

Continuous variation

[ 7 ]

Explain the reasons for variation in human height.

Concept essentials

  • Continuous variation shows a range rather than separate phenotype classes.
  • Polygenic inheritance can produce many intermediate phenotypes.
  • Environmental factors can contribute to continuous variation.