IGCSE Biology / Genes and inheritance
Ratios, and what they do not promise revision notes
Edexcel International GCSE Biology (4BI1), chapter 18: Genes and inheritance, specification point 3.25. Free lesson.
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A genetic ratio gives the probability of each possible outcome for each offspring. It does not promise what will happen in one small family or group, because fertilisation is random.
How to express a genetic ratio
For two heterozygous parents, Aa × Aa, a Punnett square gives four equally likely combinations. Three are unaffected and one is affected, so the predicted ratio is 3:1. The same result can be written as a fraction or percentage. Three combinations are unaffected: one AA and two Aa carriers. One combination is affected: aa. For an affected offspring, the probability is 1 in 4, or 25%.
What a ratio predicts for each offspring
A ratio describes the probability for each child separately. Each fertilisation is an independent random event, so the result for one child does not determine the result for another. A 3:1 ratio does not promise exactly three unaffected children and one affected child in every family of four. With four children, the outcome could range from none affected to all four affected. Use the form the question asks for: give a ratio, fraction or percentage as needed. If it asks for a probability, do not give only a ratio.
Why small samples can differ
Random fertilisation means that a small group may not match the predicted ratio exactly. This difference is expected by chance and does not, on its own, disprove the prediction. A 3:1 ratio predicts a pattern over many offspring, not an exact count in every small sample. For ten pea plants, eight tall and two short is close to 3:1, even though it is not exactly 7.5 tall and 2.5 short. With several hundred plants, the result would be expected to match the predicted ratio more closely.
Why large numbers matter
As the number of offspring increases, chance differences have less effect on the overall result. The observed results therefore tend to come closer to the predicted ratio. Small samples can deviate from the ratio by chance. Large samples are more likely to approach the predicted ratio.