Genetic information usually shows a pattern, not a guarantee. A careful SPM answer says what the data support, uses words such as linked or more likely, and names another factor that could also matter.
This lesson is the last in SPM Science genetics. It builds on explaining variation with examples, where genes and environment both shape a trait.
What is wrong with saying a gene causes a trait?
The sentence treats the gene as the only cause and as a certainty. Data rarely show either, because the same gene version can be present without the trait, and the trait can appear without that version.
The fix is to write what the numbers actually show. The data give a difference in how often the trait appears, which is an association.
Worked example: a fictional fruit study
A worked example: a school garden project records 100 tomato plants with allele A and 100 plants without it. The project counts plants that produce fruit classed as sweet.
| Group | Plants | Sweet fruit |
|---|---|---|
| With allele A | 100 | 70 |
| Without allele A | 100 | 40 |
With allele A, 70 ÷ 100 = 70% were sweet. Without it, 40%. The difference is 30 percentage points.
Careless conclusion: “Allele A makes tomatoes sweet.”
Careful conclusion: “Plants with allele A were more likely to produce sweet fruit: 70% compared with 40%. This suggests allele A is linked to sweetness, but 30 of 100 plants with A were not sweet, so other factors such as light or soil may also matter.”
The careful version quotes both numbers, uses a cautious verb and points to the plants that do not fit.
The mistake that costs marks
The common slip is to use always, causes or makes where the data only show a difference. The working looks complete, yet the conclusion claims more than the table can carry.
| Careless wording | Careful wording |
|---|---|
| Allele A makes fruit sweet | Allele A is linked to sweeter fruit |
| All plants with A are sweet | 70 of 100 plants with A were sweet |
| The gene decides the result | The gene may influence the result, along with the environment |
The fix is to check each verb against the numbers. If the data include exceptions, the verb must leave room for them.
When can a strong statement be used?
A simple cross gives a dependable expected ratio because the rules are fixed: each parent passes one allele. For Tt × Tt the expected ratio is 3 tall : 1 short, and you can say so with confidence.
Even there, the ratio is expected across a large number of offspring, not guaranteed for each family. The rule is to match the strength of the wording to the strength of the evidence.
Check yourself
A fictional study records 50 mice with variant B and 50 without it. In the first group, 40 have a darker coat. In the second, 15 have a darker coat. Write one careful conclusion.
Answer
With variant B, 40 ÷ 50 = 80% had a darker coat. Without it, 15 ÷ 50 = 30%.
Sample answer: “Mice with variant B were more likely to have a darker coat (80% compared with 30%), so the variant is linked to darker coats. Since 10 of 50 mice with B were not dark, other factors may also matter.”
What to study next
Test the whole cluster with the genetics practice set. For the general habit of judging evidence, read evaluating a science claim from a supplied dataset.
If you want a teacher to mark your wording, see online one-to-one Science tuition.