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
- SL
Continuous Variation Produces a Measurable Range
Continuous variation has many intermediate values and often results from several genes, environmental factors, or both.
| Variation | Example | Useful description |
|---|---|---|
| Continuous | Human skin colour or height | Distribution, range, mean, median and mode |
| Discrete | ABO blood group | Counts or proportions in distinct categories |
In polygenic inheritance, many loci each contribute to the phenotype; environmental conditions can add further variation, often producing a broad distribution.
For student heights, the mean uses every value, the median identifies the middle position and the mode identifies the most frequent value or interval.
Continuous does not mean entirely environmental, and discrete does not mean that only one gene is always involved.
Continuous variation
Assessment in practice
1–3 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / State / Outline.
Command terms
Identify / State / Outline / Distinguish / Explain
What earns marks
Build the answer around this relationship: Continuous variation shows a range rather than separate phenotype classes.
Representative question
Question 1
[Maximum number: 7]
Explain the reasons for variation in human height.
environment affects height;
nutrition/malnutrition affects growth rate/other example of environmental factor affecting height;
genes/alleles affect height / height is partly heritable;
polygenic / many genes influence height;
continuous variation;
normal/bell-shaped distribution of height;
some alleles (of these genes) increase height and some reduce it;
many possible combinations of alleles of these genes;
specific gene mutations/alleles cause dwarfism/extreme height;
meiosis generates variation (in height);
mutations generate variation (in height);
males tend to be/are on average taller than females;
loss of height during aging;
Retrieve the Core Inheritance Route
Core D3.2 is secure when the student can move from allele rules into predictions and evidence: gametes form genotypes, genotypes can produce phenotypes, different dominance patterns need different notation, and pedigrees or plots require evidence-based interpretation.
- haploid gametes carry one allele and fertilization restores a diploid genotype
- dominance, codominance, incomplete dominance, environment, and plasticity affect the observed trait
- PKU, ABO, sex determination, and haemophilia use different inheritance rules and notation
- pedigrees infer inheritance patterns and box plots summarize continuous variation
Solve Core Inheritance Questions
Core inheritance exam questions reward disciplined reasoning. First identify the inheritance rule, then write the correct notation or evidence, then state the phenotype, ratio, or conclusion. This prevents the common mistake of writing definitions without solving the genetic problem.
- Use allele and genotype notation correctly for monohybrid, ABO, PKU, haemophilia, and sex-determination contexts.
- Connect genotype, dominance pattern, environment, or plasticity to phenotype.
- Use pedigree or box-plot evidence to justify an inheritance or variation conclusion.
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.