Learn the idea. Then test the explanation.
Use these short activities to practise active recall and reasoning. They are introductory learning aids; combine them with the full syllabus and authentic exam problems.
Worked example: an inheritance ratio
Two plants with genotype Aa are crossed. Assume complete dominance, random fertilisation and equal survival. What fraction of their offspring will have the recessive phenotype?
- Each parent can contribute A or a with equal probability.
- The possible combinations are AA, Aa, aA and aa.
- Only aa expresses the recessive phenotype under the assumption given.
- Therefore the expected fraction is 1 out of 4, or 25%.
Think one step further: this is an expected probability. A small actual sample need not contain exactly one recessive plant in every four.
How to read an experiment
- Question: what claim is the experiment testing?
- Variable: what did the investigator deliberately change?
- Comparison: what control helps interpret the result?
- Measurement: what was measured, in what units, and how often?
- Inference: what conclusion does the evidence support—and what does it leave uncertain?
A mistake worth avoiding
Two things changing together does not by itself prove that one caused the other. Look for alternative explanations and how the experiment controls them.
Apply these ideas in practice →
Official sources · checked 20 October 2026