A group of students measured the hand span of 30 students in their class. They found that hand span showed a wide range of values between two extremes, with most students having a similar measurement. Another characteristic they recorded was blood group, which could only be A, B, AB or O. Explain why hand span is an example of continuous variation whilst blood group is an example of discontinuous variation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Hand span shows continuous variation because it can take any value between two extremes and the values are spread over a range.
Hand span is influenced by both genes and environmental factors such as nutrition and activity.
Blood group shows discontinuous variation because it can only be one of a few distinct types – A, B, AB or O.
Blood group is determined by genes alone and is not affected by the environment.
Hand span is influenced by both genes and environmental factors such as nutrition and activity.
Blood group shows discontinuous variation because it can only be one of a few distinct types – A, B, AB or O.
Blood group is determined by genes alone and is not affected by the environment.
Examiner tips
- Use the word "continuous" and "discontinuous" explicitly. Mention the range of values for hand span and the discrete categories for blood group. Show that hand span is affected by genotype and environment, while blood group is only genetic.
Common mistakes
- Saying hand span is discrete or blood group is continuous. Forgetting to mention the environmental influence on hand span. Using vague terms like "varies" without specifying continuous or discontinuous.
Mark scheme (4 marks)
- Continuous variation produces a range of phenotypes between two extremes / hand span can take any value between two extremes
- Continuous variation is affected by both genes and environmental factors / hand span is influenced by both genotype and environment
- Discontinuous variation can only appear in discrete / separate categories / blood group can only take a limited number of distinct values
- Discontinuous variation is generally influenced by genes alone / blood group is determined by genes only and not by the environment
Key terms in this question
continuous variation · discontinuous variation
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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