17.1.217-18 · Selection, Evolution, Classification and Conservation

Variation- Discontinuous and Continuous — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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17.1.2 Variation: Discontinuous and Continuous — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Distinguish discontinuous from continuous variation in terms of genetic control, data form and graph form.Variation — the differences between at least two things in level,…

Intraspecific variation
Discontinuous variation
Other examples
Caused solely by genetic factors
Caused by an interaction between genetics and the environment
Standard deviation

Part 1 of 9Variation within a species

Variation

Variation the differences between at least two things in level, presence, quantity or feature.

Intraspecific variation

Intraspecific variation differences between individuals of the same species, which is the variation relevant to natural selection.

Phenotypic variation can be qualitative or quantitative.

Qualitative differences give discontinuous variation.
Quantitative differences give continuous variation.

Part 2 of 9Discontinuous variation

Discontinuous variation

Discontinuous variation qualitative differences that fall into discrete, distinguishable categories with no intermediates.
A feature cannot lie between categories.
Examplethere are four ABO blood groups and a person has only one of them.

Identified from a table or graph by its distinct categories.

Data are qualitative or counts of categories.
Shown on a bar chart with gaps between the bars, or on a pie chart.

Other examples

Other examples presence or absence of a feature such as tongue rolling, and purple or white flower colour in peas.

Part 3 of 9Continuous variation

Continuous variation

Continuous variation quantitative differences with a full range of values between two extremes and no categories.
Examplethe mass or height of humans.
Othersleaf length and heart rate.

Identified from a table or graph by the lack of categories and the presence of a range of values.

Data are measurements, shown on a histogram with no gaps or a frequency polygon.
With a large sample the shape is often a symmetrical, bell-shaped normal distribution.

Part 4 of 9Genetic basis of discontinuous variation

Genetic basis of discontinuous variation

Caused solely by genetic factors

Caused solely by genetic factors the environment has no direct effect.

At the genetic level, different alleles at a single gene locus have a large effect on the phenotype.

A diploid organism inherits two alleles of each gene, which may be the same or different.

Typically controlled by one or a few genes, each with a large effect.

Examplethe F8 gene codes for blood clotting protein Factor VIII.
Its alleles decide whether normal Factor VIII is made and so whether the person has haemophilia.

Part 5 of 9Genetic basis of continuous variation

Genetic basis of continuous variation

Caused by an interaction between genetics and the environment

Caused by an interaction between genetics and the environment phenotype = genotype + environment.

Different alleles at a single locus have only a small effect on the phenotype.

Different genes can have the same effect on the phenotype, and these effects add together (additive effect).

A large number of genes with a combined effect on one phenotype are called polygenes.

Part 6 of 9Additive effect of genes

Additive effect of genes

Two unlinked genes H/h and T/t both affect plant height additively.

Each recessive allele (h, t) contributes x cm of height.
Each dominant allele (H, T) contributes 2x cm of height.

Total height is the sum of the contributions of all four alleles, so genotypes give a range of heights.

hhtt gives 4x cm, Hhtt or hhTt gives 5x cm, HhTt gives 6x cm.
HHTt or HhTT gives 7x cm, and HHTT gives 8x cm.

The effect of each gene is not always equal, so always check the information given about the genes concerned.

Part 7 of 9Comparing discontinuous and continuous variation

Feature	Discontinuous	Continuous
Type of difference	Qualitative	Quantitative
Categories	Distinct classes, no intermediates	Full range of values
Genes	One or few, large effect each	Many (polygenes), small additive effects
Environment	No direct effect	Large effect
Graph	Bar chart or pie chart	Histogram or normal curve
Example	ABO blood group	Human height

Part 8 of 9Describing a normal distribution

Mean

Mean shows the centre of the distribution.

Standard deviation

Standard deviation measures how spread out the data are around the mean.
A small standard deviation means values cluster close to the mean.
A large one means a wide spread, so two samples with the same mean can differ in variability.

In a normal distribution about 68% of values lie within one standard deviation of the mean and about 95% within two.

Skewed distribution

Skewed distribution has a longer tail on one side;
- a bimodal distribution has two peaks and may indicate two groups in the population.

Selection can change the distribution of a characteristic over time by shifting the mean or narrowing the spread.

Part 9 of 9More from the specification

Key terms

Term	Meaning
Discontinuous variation	Qualitative differences falling into distinct categories with no intermediates.
Continuous variation	Quantitative differences giving a full range of values between two extremes.
Intraspecific variation	Variation between individuals of the same species.
Polygene	One of many genes that together have an additive effect on one phenotype.
Additive effect	Combined effect of several genes or alleles adding together to determine a phenotype.
Normal distribution	Symmetrical, bell-shaped distribution of a continuous characteristic around the mean.
Standard deviation	A measure of how spread out data are around the mean.

Avoid these mistakes

Saying continuous variation has no genetic basis — it is polygenic as well as environmental.

Saying discontinuous variation is affected by the environment — it is determined by genes only.

Drawing a bar chart with gaps for continuous data — a histogram with no gaps (or a curve) is used for continuous data.

Assuming every gene has the same additive effect — read the effect given for each allele.

Last updated: 18 October 2026