14.3 Selective Breeding

A free GCSE Biology lesson for AQA (8461) from OBA School: how humans breed plants and animals for useful characteristics, and the problems this can cause. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe the steps of selective breeding
• Give examples of characteristics chosen by selective breeding
• Explain the risks of selective breeding, including inbreeding
• Interpret data on selective breeding and compare it with natural selection

Key words

• Selective breeding: Humans choosing which plants or animals breed, to produce offspring with a useful characteristic. Also called artificial selection.
• Desired characteristic: The feature the breeder wants, e.g. high milk yield or disease resistance.
• Generation: One round of parents producing offspring.
• Inbreeding: Breeding closely related individuals. It makes harmful inherited conditions more likely.
• Gene pool: All the different alleles present in a population.

What is selective breeding?

For thousands of years people have bred food crops from wild plants and domesticated wild animals. By choosing which individuals breed, they gradually changed them. This is selective breeding (artificial selection).

• Disease resistance in food crops; Examples Wheat or potatoes that are not damaged by a fungus
• More meat or milk from animals; Examples Dairy cows with a high milk yield; fast-growing chickens
• Gentle nature in domestic animals; Examples Calm dogs that are safe with children
• Large or unusual flowers; Examples Roses, tulips and sunflowers bred for colour and size

The process

Each time, only the best offspring are allowed to breed. Over many generations, the alleles for the desired characteristic become more common, until all the offspring show it.

Worked example: percentage increase

Use Figure 2 to calculate the percentage increase in milk yield from generation 1 to generation 6.
Increase = 7500 − 5200 = 2300 litres
Percentage increase = 2300 ÷ 5200 × 100 = 44.2%

Answer: 44.2% (1 d.p.)

Problems with selective breeding

• Inbreeding. Breeders often use closely related animals. Some breeds then become prone to disease or inherited defects (for example, some dog breeds have breathing or joint problems).
• Less variation. The gene pool shrinks. If a new disease appears, or the climate changes, there may be no individuals with alleles to resist it, so the whole crop or herd could be lost.
• Welfare. Animals bred for very fast growth or high yield can suffer, e.g. leg problems in fast-growing chickens.

Key fact: Selective breeding does not create new genes. It only changes how common existing alleles are.

Selective breeding vs natural selection

• Who or what selects?; Selective breeding Humans; Natural selection The environment (predators, climate, food supply…)
• Features chosen; Selective breeding Ones useful to humans; Natural selection Ones that help the organism survive and reproduce
• How fast?; Selective breeding Fairly fast: tens of generations; Natural selection Usually slow: many generations
• Effect on variation; Selective breeding Reduces it a lot; Natural selection Variation usually stays high

Exam tip: If a question asks you to describe selective breeding, always say: choose parents with the characteristic → breed → select the best offspring → repeat over many generations.

Common mistakes

• Saying selective breeding puts new genes into an organism. That is genetic engineering.
• Describing only one generation. The process must be repeated over many generations.
• Saying selective breeding increases variation. It reduces variation and shrinks the gene pool.
• Forgetting that the parents must already show the desired characteristic.

Revision questions

Try these, then check your answers below each one.

Question 1. Selective breeding is also called… (Choose from: artificial selection, natural selection, genetic engineering, cloning.)

Answer: artificial selection

Working: Selective breeding is artificial selection: humans choose which individuals breed.

Question 2. True or false? Selective breeding increases genetic variation in a population.

Answer: False

Working: False. Selective breeding reduces variation and shrinks the gene pool.

Question 3. What is the main risk of inbreeding? (Choose from: Offspring are more likely to inherit harmful genetic defects, The animals form a new species, New genes are added to the animals, The animals stop producing offspring at all.)

Answer: Offspring are more likely to inherit harmful genetic defects

Working: Closely related parents are more likely to pass on the same harmful alleles, so inherited defects and disease are more likely.

Related lessons

• Previous lesson: 14.2 Evolution by Natural Selection
• Next lesson: 14.4 Genetic Engineering
• In this chapter: 14.1 Variation
• In this chapter: 14.5 Cloning
• In this chapter: 14.6 Adult Cell Cloning

Continue in the full course

This lesson is section 14.3 (AGCBH1403) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.