Lesson 10 of 11 / Inheritance
Continuous or discontinuous variation?
Should these observations form a range or separate categories?
I can distinguish continuous and discontinuous variation and interpret suitable examples.
I can distinguish continuous and discontinuous variation.Syllabus
Syllabus K325, Topic 14(j). Distinguish continuous and discontinuous variation.
About 5 min
Make a guess. You are not marked.
Is human height continuous or discontinuous variation?
Show the answer
Continuous: there is a full range of values.
Heights can take any value in a range. Many genes and the environment both play a part.
The key idea
Continuous variation has a range of intermediate values; discontinuous variation has distinct categories with no intermediate category between them.
Number of individuals
Height is continuous: values can lie between the marked measurements. Binning them into adjacent 10 cm intervals does not remove intermediate values.
| Height (cm) | Individuals |
|---|---|
| 140 to below 150 | 2 |
| 150 to below 160 | 5 |
| 160 to below 170 | 8 |
| 170 to below 180 | 6 |
| 180 to below 190 | 3 |
Original illustrative datasets, not population estimates. Height bars represent equal-width measurement intervals; ABO bars represent distinct categories. The number above each bar is its count.
Explanation
Variation means differences among individuals. Continuous variation includes characteristics such as height, with many intermediate values across a range. Measurements may be grouped into intervals for a graph, but the underlying characteristic is still a continuous measurement.
Discontinuous variation has distinct categories, such as the ABO blood groups A, B, AB and O. There is no blood group halfway between A and B in this classification; AB is its own category produced by codominance.
Many continuous characteristics are influenced by several genes and by the environment. Height, for example, reflects inherited factors and environmental influences such as nutrition. ABO group is determined by inherited alleles.
Choose a graph that matches the data. A histogram can group continuous measurements into intervals with adjacent bars. A bar chart can compare separate categories with gaps. Grouping heights into short and tall for a classroom table does not turn the underlying trait into a naturally discontinuous one.
Step by step
- 1
Ask whether intermediates exist
Height can lie between two measurements; ABO categories do not blend continuously.
- 2
Name a suitable example
Give a characteristic rather than only an individual observation.
- 3
Describe the evidence appropriately
Use measured intervals for continuous data and categories for discontinuous data.
Worked example
A table of height groups
A student groups heights into below 150 cm and 150 cm or above. Has height become discontinuous variation?
One way to explain it
No. The student has imposed categories on a continuous characteristic. Heights still have intermediate measurable values across a range.
Why this answer works
- Separate the underlying trait from the chosen data grouping.
- Look for possible intermediate values rather than just the number of table headings.
Watch out for this
A student says: "Any characteristic shown in a bar chart must be discontinuous." What is wrong with this?
Show the answer
The graph choice alone cannot establish the type. Identify whether the underlying characteristic has a continuous range or distinct categories.
Check your understanding
Which comparison is correct?
Show the answer
Height is continuous; ABO blood group is discontinuous.
Height has intermediate measurements, while ABO has distinct categories.
Quick recall
Should these observations form a range or separate categories?