Genes decide what an organism can become. The environment decides how much of that is reached.
This lesson is part of variation. It builds on comparing continuous and discontinuous variation.
How do you tell the two causes apart?
Keep one of them the same and let the other change. The difference in outcome then shows which cause is responsible.
| If the genes are the same | The difference is due to |
|---|---|
| Identical twins reared apart | Environment |
| Cuttings from one plant in different light | Environment |
| Different varieties in identical conditions | Genes |
Worked example: a cutting experiment
A student takes six cuttings from one balsam plant. Three are grown in full sunlight and three in a shaded corner. After four weeks, the sunlit cuttings have a mean height of 18 cm and the shaded ones 27 cm.
- The cuttings share the same genes, because they came from one plant.
- Only the light differs between the two groups.
- The 9 cm difference (27 − 18) must be due to the environment, not the genes.
Answer: “The cuttings are genetically identical, so the height difference is caused by the environment. In shade, the plant grows taller to reach light.”
Check: 27 − 18 = 9 cm.
How do you write a balanced answer?
Most human traits depend on both causes. A balanced answer names one genetic point and one environmental point.
Use this template: “Genes set the range for [feature], for example [gene effect]. The environment, such as [factor], affects how far the individual reaches that range.”
For height: genes set the potential to be tall. Nutrition and health affect whether that height is reached.
The mistake that loses marks
The common slip is to write “it is genetic, so the environment does not matter”. Most features are shaped by both.
| Statement | Problem | Better |
|---|---|---|
| Height is only genetic | Ignores nutrition | Height is influenced by genes and diet |
| Blood group depends on diet | Wrong cause | Blood group is controlled by genes |
| Twins are identical in every way | Environment still acts | Twins have the same genes but may differ through environment |
Check yourself
Two tomato plants from the same parent plant are grown, one in rich soil and one in poor soil. The first is larger. State the cause and explain the reasoning.
Answer
The cause is the environment. Both plants came from the same parent, so their genes are the same. The only difference is the soil, so the nutrients in it explain the difference in size.
What to study next
Continue with interpreting variation distributions. The simple inheritance cross explorer shows how genes are passed on, and the variation practice set tests the whole chapter.
If a teacher could test your balanced answers on new traits, see online one-to-one Biology tuition.