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B13.3 Selective Breeding

GCSE Combined Science · AQA · Chapter B13: Variation and Evolution

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: how humans breed plants and animals for useful characteristics, and the risks of inbreeding. 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 (artificial selection)
  • Give examples of characteristics chosen by selective breeding
  • Explain the benefits and risks of selective breeding, including inbreeding and a smaller gene pool
  • 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 desired 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 some breeds prone to disease or inherited defects.
  • 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 slowly changed them. All dog breeds, from tiny terriers to huge Great Danes, are one species bred from wolves in this way. This is selective breeding, also called artificial selection.

  • Disease resistance in food crops; Examples wheat or barley varieties that resist a fungus
  • More meat, milk or eggs; Examples dairy cows bred for high milk yield; hens that lay more eggs
  • Gentle nature in domestic animals; Examples calm, friendly dogs that are safe with children
  • Large or unusual flowers; Examples roses and orchids bred for scent, colour and size
  • Crops that suit the weather; Examples short-stemmed wheat that is not flattened by wind and rain

The process

Each generation, 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 the bar chart to calculate the percentage increase in eggs per hen from generation 1 to generation 6.
Increase = 279 − 182 = 97 eggs
Percentage increase = 97 ÷ 182 × 100 = 53.3%

Answer: 53.3% (1 d.p.)

Benefits and risks

  • more food from the same land or animals; Risks inbreeding: some breeds become prone to disease or inherited defects (e.g. hip or breathing problems in some dogs)
  • crops that resist disease or survive bad weather; Risks the gene pool shrinks: less variation, so a new disease or a change in climate could affect the whole population
  • animals that are easier and safer to keep; Risks animal welfare: very fast growth or high yield can cause pain or ill health

Worked example: a small gene pool

A rare breed of cattle has 4 000 animals, but they are so closely related that their gene pool is like that of only 80 unrelated animals. What fraction is this?
80 ÷ 4 000 = 1/50 = 0.02, i.e. 2%.

Answer: the gene pool is only 2% of what the numbers suggest

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
  • Effect on variation; Selective breeding reduces it a lot; Natural selection variation usually stays high

Exam tip: To describe selective breeding, always include: choose parents with the characteristic → breed them → 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 also called artificial selection, because humans, not the environment, choose which organisms breed.

Question 2. The diagram shows the steps of selective breeding. What should go in the box marked "?" (Choose from: Choose the offspring that best show the characteristic, Add a gene from another species, Let all the offspring breed at random, Wait for a mutation in every animal.)

Answer: Choose the offspring that best show the characteristic

Working: After breeding, the breeder selects the offspring that show the desired characteristic best, and uses them as the next parents. This is repeated for many generations.

Question 3. True or false? Selective breeding increases the genetic variation in a population.

Answer: False

Working: Breeding only from chosen, often related, parents reduces variation and makes the gene pool smaller.

Related lessons

Continue in the full course

This lesson is section B13.3 (AGCSC1303) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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