IGCSE · Practice questions · Selective breeding

Select, cross, repeat.

Six original Cambridge-style questions on selective breeding: the steps, wheat and cattle examples, yield data over generations and (Extended) the differences between natural and artificial selection.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Humans choose the parents.

Step 1: select individuals with the desirable feature.
Step 2: cross them to produce the next generation.
Step 3: select offspring showing the feature; repeat over many generations.
Extended: natural selection is by the environment for survival; artificial selection is by humans for usefulness, and usually faster.

Recall

[3 marks]

Describe how selective breeding is carried out.

Humans select individuals with the desirable feature. ✓
These individuals are crossed (bred together) to produce the next generation. ✓
Offspring showing the desirable feature are selected and bred, and this is repeated over many generations. ✓

Explain

[4 marks]

A plant breeder wants a variety of tomato plant that is resistant to a fungal disease. Outline how she could produce it by selective breeding.

Grow many tomato plants and expose them to the fungus (or find plants that are not infected). ✓
Select the plants that show resistance to the disease. ✓
Cross these plants and grow the seeds; expose the offspring to the fungus again. ✓
Select the most resistant offspring to breed, and repeat for many generations. ✓

Data

[4 marks]

The table shows the mean milk yield of cows in a herd that was selectively bred.

Generation	0	2	4	6
Mean yield / litres per day	12	16	19	21

(a) Calculate the percentage increase in mean yield from generation 0 to generation 6. (b) Describe the pattern. (c) Suggest why the increase is smaller in later generations.

(a) (21 − 12) ÷ 12 × 100 = 75%. ✓
(b) The yield increases every generation, but by less each time (4, then 3, then 2 litres per day over two generations). ✓
(c) Most cows now carry the alleles for high yield, so there is less genetic variation left to select from. ✓
Yield may also be limited by the environment, such as the amount of food the cows can eat. ✓

Explain

[3 marks]

A farmer feeds his best cow extra food so that it gives more milk. Explain why breeding from this cow because of its extra milk may not improve the milk yield of its offspring.

The extra milk is caused by the environment (more food), not by its genes. ✓
Only inherited (genetic) variation is passed on to offspring. ✓
So the offspring may not inherit any alleles for a high milk yield. ✓

Analysis

[4 marks]

Extended. Outline four differences between natural selection and artificial selection.

Natural selection: the environment selects; artificial selection: humans select. ✓
Natural: features that help survival and reproduction; artificial: features useful to humans. ✓
Natural selection is usually slower; artificial selection is usually faster. ✓
Natural selection leads to adaptation to the environment; artificial selection produces new varieties and breeds, which may be less able to survive in the wild. ✓

Explain

[3 marks]

Extended. Suggest one disadvantage of selectively breeding crop plants for a single feature, such as high yield, over many generations.

Breeding only from a few selected plants reduces genetic variation in the crop. ✓
If a new disease or a change in climate occurs, there may be no plants with alleles for resistance. ✓
So the whole crop could be lost (or the crop may depend on fertiliser and pesticides). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.