Selective breeding

In selective breeding, people choose the plants or animals with the characteristic they want, breed them, choose the best of the offspring and breed those, over many generations. That is how we have crops that resist disease, cows that give more milk and gentle dogs. Breeding close relatives over and over reduces variation, which can bring inherited defects and leave them all open to a new disease.

Key facts

  • Selective breeding: choose parents with the characteristic you want, breed them, choose the best offspring and breed them, and repeat over many generations. Examples are disease-resistant crops, cows that give more milk and gentle dogs.
  • Selective breeding can lead to inbreeding: there is less variation, so the organisms are more likely to suffer from inherited defects and from new diseases.

Worked examples

Example 1

Put the steps of selective breeding in order.

  1. Choose parents with the characteristic you want.
  2. Breed them together.
  3. Choose the offspring that show the characteristic best.
  4. Breed these, and repeat over many generations.

Answer: choose parents with the characteristic you want → breed them together → choose the offspring that show the characteristic best → breed these, and repeat over many generations

Example 2

A farmer wants cows that give more milk. Describe how she could use selective breeding.

  1. Choose the cows that give the most milk.
  2. Breed them with a bull whose mother gave a lot of milk.
  3. From the offspring, choose the cows that give the most milk.
  4. Breed these, and repeat for many generations until the yield is high.

Answer: breed the cows that give the most milk with bulls from high-yield mothers, choose the best offspring and repeat over many generations

Example 3

Give two characteristics that farmers selectively breed crop plants for.

  1. Farmers choose plants with useful characteristics and breed them together.
  2. A high yield means more food from each plant.
  3. Resistance to disease means fewer plants are lost.

Answer: for example: a high yield and resistance to disease (also: larger fruit, or coping with drought)

Example 4

Which technique produced these: wheat with bigger grains, bred from the best plants each year?

  1. Parents with the characteristic were bred over many generations.
  2. So this is selective breeding.

Answer: selective breeding

Example 5

Explain why selective breeding can cause problems for animals.

  1. Breeders keep breeding from a few closely related animals.
  2. This reduces the number of different alleles: inbreeding.
  3. Harmful recessive alleles are more likely to pair up, and with little variation a new disease could affect them all.

Answer: it reduces variation in the gene pool, so inbreeding makes inherited disorders more likely and the whole population could be wiped out by a new disease

Example 6

What is the main problem caused by selective breeding?

  1. Breeding closely related organisms reduces variation.
  2. A new disease could affect them all, and harmful recessive alleles show up more often.

Answer: inbreeding, so there is less variation and more risk from diseases and inherited defects

Example 7

After 5 generations of selective breeding, the mean milk yield of a herd rose from 4,000 litres to 5,200 litres a year. Calculate the percentage increase.

  1. Increase: 5200 − 4000 = 1200 litres
  2. Percentage: 1200 ÷ 4000 × 100 = 30%

Answer: 30%