Variation means that individuals of one species are not all the same. It comes from genes inherited from parents and from the environment, and most characteristics involve both.
This lesson belongs to SPM Science genetics. It follows completing basic inheritance diagrams, since different gametes meeting is one source of difference.
How can I tell which cause a question wants?
Ask what stayed the same and what changed. If the individuals share the same genes and differ, the cause is the environment. If they share the environment and differ, the cause is genes.
A short rule helps: same genes, different results point to the environment; same environment, different results point to genes.
Worked example: a table of seedlings
A teacher collects six seeds from one invented plant, so the seeds are almost identical genetically. Three grow in one place and three in another.
| Seedling | Place | Light | Height after 3 weeks (cm) |
|---|---|---|---|
| 1 to 3 | Window sill | Bright | 9, 10, 11 |
| 4 to 6 | Cupboard shelf | Dim | 14, 15, 16 |
The mean height on the sill is (9 + 10 + 11) ÷ 3 = 10 cm. In the cupboard it is (14 + 15 + 16) ÷ 3 = 15 cm.
The seedlings share genes, so the 5 cm difference in mean height points to the environment, in this case light. A good answer adds the evidence: “The seedlings in dim light grew taller than those on the sill, so light affected growth.”
A careful student also adds a limit. Within each group, heights still differ by 2 cm, which suggests small genetic or other differences remain.
The mistake that costs marks
The usual slip is to give a one-cause answer to a two-cause question. A student writes “variation is caused by genes” every time, which fits some cases and is wrong for this one.
| Situation | Wrong | Right |
|---|---|---|
| Identical twins differ in weight | Different genes | Same genes, so a different environment such as diet |
| Two siblings have different blood groups | Environment | Different combinations of alleles from the parents |
| Seedlings of one plant differ in height | Genes only | Environment, here light, plus small genetic differences |
The fix is to name both sources first, then choose the one the evidence supports.
Variation and cross diagrams
Variation from genes appears in a cross diagram as different genotypes. The 1 TT : 2 Tt : 1 tt result shows that one pair of parents can produce offspring with three genotypes.
That is why siblings differ: each gamete carries one of the parent’s alleles at random, and each fertilisation joins a different pair. This is the link between the diagram skill and this lesson.
Check yourself
Two identical twins live in different towns. One twin is 4 cm taller than the other. Explain the cause of the difference, and give one environmental factor.
Answer
Identical twins have the same genes, so the genetic cause can be ruled out. The difference comes from the environment.
One example is diet, such as different food or nutrients during growth. Another is the amount of exercise.
What to study next
The final lesson applies these ideas to claims. Continue with interpreting genetic information without deterministic claims, then test yourself on the genetics practice set.
If you want a teacher to push your explanations until they name a cause, a link and an example, see online one-to-one Science tuition.