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Variation in Living Things

Variation means the differences between living things of the same species. Continuous variation has every value in a range (height, mass); discontinuous variation falls into clear groups (blood group, flower colour). Variation is caused by genes, by the environment, or by both. Only changes in genes are heritable. People use variation in selective (and hybrid) breeding, and genetic modification adds a useful gene directly.

🎬 Step-by-step story

  1. Ten plants of one kind stand in a row. Look closely: no two are the same height. These differences are called variation.
  2. Line them up from shortest to tallest. Every height in between is possible, so they make a smooth ramp. This is continuous variation.
  3. Now look at flower colour. Each flower is purple or white, nothing in between. Clear groups like this are discontinuous variation.
  4. Two seeds with exactly the same genes. One grows in sunlight, one in shade. The sunny one grows tall. This difference comes from the environment and will not pass to its seeds.
  5. Selective breeding: in each generation keep only the tallest plants and grow their seeds. Watch the average height rise.
  6. Free play: change the sunlight, breed more generations, or add a pest-resistance gene to make a GM plant.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

If they are the same species, why are they different?

Each one got a new mix of genes from its parents and grew in slightly different conditions. Step 1 shows ten plants of one kind, all different heights.

Height is measured in whole cm. Doesn't that make it discontinuous?

No. We round, but real heights can be any value between. The smooth ramp in step 2 shows there are no gaps.

Why can't a flower be 'half purple'?

Flower colour here is controlled by a gene with two forms, so there are only two clear groups. See step 3.

How do we know the difference came from the environment?

The two seeds have the same genes. Only sunlight is different, so sunlight caused the height difference.

Does training your muscles make your children strong?

No. Training changes your body, not your genes. Only gene-caused variation is passed on, like the breeding in step 5.

Why does the average rise in selective breeding?

Only the tallest plants become parents, and height is partly in the genes, so their seeds tend to be tall too.

How is GM different from breeding?

Breeding mixes whole sets of genes over many generations. GM puts one chosen gene in directly; try the GM button in free play.

What is variation?

Variation is the set of differences between living things. Differences between two different species (a cat and a dog) are large. Differences inside one species are smaller: every person, dog or mango tree is a little different.

Variation matters. It helps a species survive when its surroundings change, and it lets farmers choose the best plants and animals.

Continuous and discontinuous variation

Continuous variation: the feature can take any value in a range, like height, mass, hand span or leaf length. A bar chart of a class's heights makes a smooth hill shape. Usually many genes plus the environment cause it.

Discontinuous variation: the feature falls into a few clear groups with nothing in between, like blood group, eye colour type, tongue rolling or pea flower colour. A bar chart has separate bars. Usually one or a few genes cause it, and the environment has little effect.

Causes of variation: genes and environment

Genetic causes. Each child gets half its genes from each parent. Sexual reproduction mixes the genes in a new way each time, so brothers and sisters are different. Rarely, a gene changes by itself; this is a mutation, a new source of variation.

Environmental causes. Food, sunlight, water, temperature, exercise, illness and accidents change how a body grows. A plant in shade stays short; a person who trains gets stronger; a scar comes from an injury.

Most features come from both: your genes set a possible range of height; food and health decide where in that range you end up.

Heritable and non-heritable variation

Heritable variation is caused by genes, so it can pass to the next generation (natural hair colour, blood group). Non-heritable (acquired) variation is caused only by the environment and is not passed on (a scar, dyed hair, muscles from training, a plant made short by shade). Test: if the change is only in the body and not in the genes of the sex cells, the children will not get it.

Using variation: selective and hybrid breeding

Selective breeding (artificial selection): people choose parents with the feature they want, breed them, and repeat for many generations. Examples: cows that give more milk, wheat with short strong stems, many dog breeds from one wild ancestor.

Hybrid breeding: cross two different pure varieties, each with a good feature, to get offspring with both (for example high yield + disease resistance). Hybrid rice raised harvests in many Asian countries.

Limits: it is slow, and breeding only from a few 'best' parents reduces variation, so the whole crop can catch the same disease.

Genetic modification (GM)

Genetic modification means scientists cut a useful gene from one organism and put it into another, even of a different species. Examples: insect-resistant cotton with a bacterial gene, rice with extra vitamin A, bacteria that make human insulin.

Benefits: faster than breeding, can add features breeding cannot. Concerns: effects on wild plants and insects, safety testing, cost of seeds for farmers. Countries have laws to test and label GM foods.

Try it: measure your class

Measure the hand span (thumb tip to little finger tip) of 15 people in cm. Group them: 15–16, 17–18, 19–20 and so on, and draw a bar chart. You will see a hill shape: continuous variation. Then ask the same people: can you roll your tongue? Draw two bars: yes and no. That is discontinuous variation.

Key formulas and definitions

Worked examples

1. Sort these into continuous or discontinuous: height, blood group, mass, tongue rolling.

Continuous: height, mass (any value in a range). Discontinuous: blood group, tongue rolling (clear groups).

2. Identical twins have the same genes, but one is heavier. Why?

Same genes, so the difference must come from the environment: food, exercise or illness. It is non-heritable.

3. A farmer wants sheep with thicker wool. Describe how.

Choose the ram and ewes with the thickest wool, breed them, keep the lambs with the thickest wool, breed those, and repeat for many generations.

4. Why is a scar not passed to children?

A scar changes skin cells only. It does not change the genes in sperm or egg cells, so it is non-heritable.

Common mistakes

Practice quiz

1. Which is discontinuous variation?
2. Which is non-heritable?
3. A plant in shade grows short. This is caused by:
4. Choosing the best parents for many generations is:
5. Putting a bacterial gene into cotton is:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What are the two types of variation?

Continuous variation (any value in a range, like height) and discontinuous variation (clear groups, like blood group).

What causes variation?

Genes inherited from parents (mixed in sexual reproduction and changed by mutation) and the environment, often both together.

Is selective breeding the same as genetic modification?

No. Selective breeding chooses parents over many generations; GM moves a gene directly, even from another species.

Where this is taught

Ukraine9 класHeredity and variation
China八年级（初二）U6 Ch.2 Heredity and variation

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