These questions ask you to identify the cause of a difference, classify its pattern, or explain why a lifetime change is not inherited. Use separate checks for cause, data shape and what reaches the gametes.
Classify the difference in three passes
First confirm that the comparison is within one species. Next ask where the difference comes from: alleles received at fertilisation, conditions experienced during life, or an interaction between the two. Finally inspect the pattern. Values spread across a measurable range are continuous; named groups with no intermediate values are discontinuous. The cause and the pattern are related, but they are not interchangeable labels.
| Evidence | Best conclusion |
|---|---|
| A fixed category that does not change with lifestyle | Treat it as inherited and discontinuous unless other information contradicts that classification. |
| A measurement with many intermediate values | Treat it as continuous; many genes commonly contribute, and conditions may shift the measured value. |
| A characteristic built, learned or marked during life | Treat it as acquired. It changes the individual without rewriting the alleles in that individual's gametes. |
| The same alleles but different measured outcomes | Look for environmental differences, because inherited variation has been held constant. |
| Recorded category names rather than measurements | Group the data by category; measured data instead belongs in ranges. |
For a characteristic influenced by both causes, genes set part of the biological potential and the environment affects how that potential develops. This does not mean the two causes contribute equally in every person. It means that neither cause alone explains the full spread of results.
Mistakes to screen out
| Tempting idea | Why it fails |
|---|---|
| Calling differences between separate species variation | The term here compares individuals belonging to the same species. Check the comparison group before classifying a cause. |
| Passing trained muscles or another acquired feature to offspring | A change in the body does not alter the allele set carried by eggs or sperm. Offspring inherit alleles, not a parent's lifetime alterations. |
| Swapping continuous and discontinuous | Ignore how familiar the characteristic seems. Ask whether intermediate values can exist or whether the results occupy separate groups. |
| Explaining a continuous range with one gene alone | Many small genetic effects, often combined with environmental effects, produce the numerous intermediate values in a range. |
| Letting the environment move someone between discontinuous categories | A truly discontinuous inherited category has boundaries that lifestyle does not slide across. |
Try it
Which of these characteristics is fixed by inherited alleles alone?
- The mass of muscle a person has built up through training
- How tall a person grows
- A person's blood group
- How dark a person's skin becomes after a summer outdoors
A class records data on several characteristics of its members. Which statements about grouping that data are true? Select all that apply.
- Data on a characteristic with separate categories is grouped by category name
- Measured values such as hand span are best listed as separate named categories with nothing in between
- Blood group data is best grouped into ranges of values
- Data on a measured characteristic is grouped into ranges of values
Identical twins carry the same alleles as each other. One twin has a noticeably higher body mass than the other. What must be causing that difference?
- A difference in the alleles they carry
- A mutation that one twin inherited and the other did not
- A difference in the conditions each of them has lived in
- The fact that one twin was born a few minutes earlier