Continuous vs Discontinuous Variation
Continuous variation shows a smooth range of values with no clear-cut categories, such as human height, and is affected by both genes and environment. Discontinuous variation shows a small number of distinct categories with no intermediates, such as ABO blood group, and is usually controlled by genes alone.
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What variation means
Variation refers to the differences that exist between individuals of the same species. These differences arise from genetic causes (different alleles inherited), environmental causes (diet, climate, exercise), or a combination of both.
Variation is grouped into two types based on how the values are distributed: continuous and discontinuous.
Continuous variation
Continuous variation shows a smooth, unbroken range of values between two extremes, with no distinct categories, any value within the range is possible. It usually results from the combined effect of many genes (polygenic inheritance) together with environmental influences, which is why identical genotypes can still show a range of outcomes.
- Examples: human height, body mass, skin colour, hand span, crop yield.
Discontinuous variation
Discontinuous variation shows a small number of distinct, separate categories, with no intermediate values between them. It is usually controlled by one or a few genes with limited environmental influence, so the categories stay sharply separated regardless of surrounding conditions.
- Examples: ABO blood group, ability to roll the tongue, earlobe attachment, sex.
Comparing the two
| Feature | Continuous variation | Discontinuous variation |
|---|---|---|
| Distribution of values | Smooth range, no fixed categories | Small number of distinct categories |
| Main cause | Many genes plus environment | One or few genes |
| Graph shape | Bell-shaped (normal distribution) curve | Bar chart with separate, distinct bars |
| Example | Height | Blood group |
Worked problem 1, easy: classifying characteristics
Question. Classify each of the following as continuous or discontinuous variation, giving a reason for each: (i) tongue-rolling ability, (ii) leaf length in a tree, (iii) ABO blood group, (iv) body mass in adults.
Model answer. (i) Tongue-rolling ability is discontinuous, a person can roll the tongue or cannot, with no in-between. (ii) Leaf length is continuous, leaves show a smooth range of lengths.
(iii) ABO blood group is discontinuous, there are four distinct groups (A, B, AB, O) with no intermediates. (iv) Body mass is continuous, it varies smoothly and is influenced by both genes and diet.
| Characteristic | Type of variation | Reason |
|---|---|---|
| Tongue rolling | Discontinuous | Two clear categories, no in-between |
| Leaf length | Continuous | Smooth range of values |
| ABO blood group | Discontinuous | Four distinct groups |
| Body mass | Continuous | Smooth range, affected by diet |
Worked problem 2, medium: reading a data set
Question. A class measured the heights of 40 students and grouped them into ranges, as shown. (a) State the type of variation.
(b) Name the graph best used to display the data and describe its expected shape.
Model answer. (a) Height is continuous variation: the values form a smooth range grouped into class intervals, and no student falls into a fixed either/or category. (b) The data are best shown as a histogram (or a frequency curve), and its shape is a bell-shaped normal distribution, most students near the middle height and progressively fewer at the shortest and tallest ends.
| Height range (cm) | Number of students |
|---|---|
| 145-150 | 3 |
| 151-156 | 9 |
| 157-162 | 16 |
| 163-168 | 9 |
| 169-174 | 3 |
Worked problem 3, SPM-level: genes and environment
Question. Two genetically identical plants (clones) are grown from cuttings of the same parent. One is grown in bright light and reaches 60 cm; the other is grown in shade and reaches 35 cm. (a) State the type of variation shown by plant height.
(b) Explain how two identical genotypes can produce different heights.
Model answer. (a) Plant height shows continuous variation. (b) The two plants have the same genotype, so the difference in height cannot be genetic.
It is caused by the environment: the plant in bright light carries out more photosynthesis and produces more food for growth, while the shaded plant makes less. This shows that continuous variation results from genes and environment acting together, not genes alone.
Practice problems
- 1. State whether human skin colour shows continuous or discontinuous variation, and give the reason.
- 2. A survey records how many students can and cannot roll their tongue. State the type of variation and the graph best used to display it.
- 3. Give one human characteristic caused mainly by genes alone and one caused by both genes and environment.
Practice answers
How it is examined
SPM questions may give a data set or a described characteristic and ask you to classify it as continuous or discontinuous, sketch the expected graph shape, or explain why one is more influenced by environment than the other.
Common mistakes
Source:SRC-DSKP-EN
Frequently asked questions
What is the difference between continuous and discontinuous variation?
What graph shape represents continuous variation?
Why is discontinuous variation less affected by environment than continuous variation?
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