Lesson focus: Continuous and discontinuous variationDifferences that fall into separate groups with nothing in between, such as blood group. describe distributions of measured phenotypes, not mutually exclusive genetic mechanisms.
Display and interpret continuous and discontinuous data using appropriate graphs, summary statistics and sampling decisions.
Build on earlier learning
VariationThe previous lesson examined causes of variation. This lesson examines the patterns those differences form in data.
Practical
Measuring variation
- Measure a continuous feature, such as hand span, for the class.
- Record a discontinuous feature, such as tongue rolling.
- Group the continuous data into classes.
Record: A histogram for the continuous data and a bar chart for the discontinuous data.
You will be successful when you can…
- Distinguish continuous and discontinuous patterns.
- Choose histograms or bar charts appropriately.
- Calculate range and mean.
- Evaluate sampling and measurement effects.
Words that carry the science
- continuous variation
- Variation showing a range of measured values with many possible intermediates.
- discontinuous variation
- Variation whose observations fall into distinct categories.
- histogram
- A graph of frequency across adjoining numerical intervals.
- frequency
- The number or proportion of observations in a category or interval.
- mean
- The sum of values divided by the number of values.
- range
- The difference between the maximum and minimum values.
Choose graphs that preserve the structure of variation
01Continuous traits fill a measurement scale
Height, hand span and leaf length commonly show many intermediate values. They are often influenced by many genes and environmental conditions, though a continuous pattern alone does not prove a particular mechanism.
Measurement resolution can make continuous data appear stepped. Larger, representative samples usually reveal more of the underlying distribution.
02Discontinuous traits form categories
ABO blood group falls into distinct categories. Some phenotypes with a simple genetic component are discontinuous, but categories may still depend on several genes or classification rules.
Use separated bars for categories. Use a histogramA bar graph with no gaps between the bars, used for measurements grouped into ranges. for continuous numerical intervals because adjacent bins represent neighbouring values.
03Sampling changes the observed distribution
Define the population, sample without obvious bias and standardise the method. Age, sex, developmental stage and environmental conditions can shift a distribution.
Means can hide skew or subgroups, while rangeThe spread from the lowest to the highest value, such as the values an instrument can measure or the values in your results. is sensitive to extremes. Plotting the full distribution and reporting sample size gives stronger evidence.
Continuous and discontinuous variation
Continuous variation can take any value in a range, like height: plot it as a histogram. Discontinuous variation falls into distinct groups, like blood group: plot it as a bar chart with gaps.
Controls
Values are illustrative (random samples). Bigger samples give a smoother, more reliable pattern.
Summarise a continuous sample
Five leaf lengths are 42, 48, 50, 55 and 65 mm.
- Sum the values: 42 + 48 + 50 + 55 + 65 = 260 mm.
- Divide by five: meanAn average found by adding up all the values and dividing by how many values there are. = 52 mm.
- Range = 65 − 42 = 23 mm.
- Report both statistics with the sample size and measurement unit.
How to make the reasoning visible: Mean describes the centre while range gives a simple measure of spread; neither replaces the raw distribution.
Choose a display for variation data
Match the variable to its data structure before choosing a graph.
| Variable | Data form | Appropriate display | Reason |
|---|---|---|---|
| Leaf length / mm | Continuous numerical | Histogram or dot plot | Many ordered values |
| ABO blood group | Categories | Bar chart | Distinct unordered groups |
| Number of petals | Discrete count | Bar chart or dot plot | Whole-number values |
| Hand span / cm | Continuous numerical | Histogram | Measured scale |
| Seed colour class | Categories | Bar chart | Defined classes |
Graph skills: SLAPUK and choosing a scaleDiscontinuous data: bar chart or line graph?
Why should histogram bars touch?
Why might a small sample make continuous variation look discontinuous?
Check your understanding
Answer from memory, then check against the suggested answer.
Give one example of continuous variation.
Which graph is usually used for continuous grouped data?
How is range calculated?
A student uses separated bars for height intervals. What should change?
Explain why a strongly genetic trait can still show continuous variation.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Variation can be continuous or discontinuous.
Compare continuous and discontinuous variation, with an example of each.
(2)State how each type should be displayed, and one limit of such graphs.
(3)Total for this question: 5 marks
Command word: For compareA command word: say how two or more things are similar and how they are different., pair the data pattern with the correct display and one justified example.