Selective breeding

Selective breeding, also called artificial selection, is when humans choose which plants or animals reproduce so that their offspring have a characteristic people want. Breeders pick parents that show the characteristic, breed them, then pick the best offspring and breed those, generation after generation, until all the offspring show it.

What is selective breeding?

AQA's GCSE specification defines selective breeding (artificial selection) as the process by which humans breed plants and animals for particular genetic characteristics. It works because many characteristics are partly controlled by genes, and offspring inherit genes from their parents. If only individuals with a wanted characteristic are allowed to breed, the alleles behind it become more common in each generation.

People were breeding crops and animals this way long before anyone knew about genes. Most of the crops we eat and the farm animals and pets we keep have been changed by it.

The selective breeding process

1. Decide on the characteristic you want, such as higher yield or a calm temperament.
2. Choose parents from a mixed population that show that characteristic most strongly.
3. Breed them together.
4. Choose the offspring that show the characteristic best, and breed those together.
5. Repeat over many generations, until all the offspring show the characteristic.

It takes many generations because each round only shifts the population a little, and the offspring of two good parents still vary. That is why it is slow for animals that take years to grow up, and quicker for crops with a short life cycle.

Examples of selective breeding

AQA gives four kinds of characteristic that are chosen for usefulness or appearance:

Other common targets include crops that grow faster, cope with drought or cold, or have bigger, tastier fruit, and animals that grow quickly or produce more wool or eggs.

Example: describing how to breed for a characteristic

A farmer wants a flock of sheep with thicker wool. A full answer:

1. Select a ram and ewes with the thickest wool from the flock.
2. Breed them together.
3. From their lambs, select those with the thickest wool when they are old enough.
4. Breed those together, and keep repeating for many generations.
5. Eventually the whole flock has thick wool.

Benefits and risks of selective breeding

AQA asks you to explain the benefits and risks of selective breeding given appropriate information, and to consider related ethical issues.

Why inbreeding is a problem

Breeding from a small group of closely related individuals means offspring are more likely to inherit two copies of the same recessive allele. If that allele causes a disorder, more animals will have it. AQA notes that selective breeding can lead to inbreeding, where some breeds are particularly prone to disease or inherited defects.

Why less variation matters

If every plant in a crop is very similar genetically, a new disease that can attack one can attack them all. A more varied population is more likely to contain some individuals that resist it.

Breeders reduce these risks by choosing several parents rather than one pair, and by avoiding breeding close relatives together, so that more variation is kept in the population.

Selective breeding vs natural selection vs genetic engineering

AQA describes genetic engineering as a process which involves modifying the genome of an organism. Selective breeding can only combine alleles that already exist in the species, so it is slower and less precise, but it doesn't move genes between species. You can watch selection pressures change a population over generations in the natural selection simulator.

Common mistakes

• Saying it happens in one generation. It takes many generations of choosing and breeding.
• Confusing it with genetic engineering. Selective breeding doesn't change genes directly or add genes from other species.
• Saying the animals "learn" or "adapt". The change happens because of which alleles are passed on, not because individuals change.
• Forgetting to say "breed the offspring". Choosing parents once isn't enough; the best offspring must be selected and bred again.
• Missing the risk of inbreeding in a benefits and risks question.

How characteristics are inherited is in the Genetics and DNA course. For practice in a short timed paper, try today's GCSE Biology Higher or GCSE Combined Science Higher paper.

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