IGCSE Biology / Selective breeding
Breeding animals and crops for a purpose revision notes
Edexcel International GCSE Biology (4BI1), chapter 20: Selective breeding, specification point 5.10, 5.11. Free lesson.
Watch the video lesson · IGCSE Biology past papers
Selective breeding is a cycle in which humans choose which organisms reproduce to increase a desired characteristic. It uses variation already present in a population, but can reduce genetic variation and increase the risk of harmful alleles appearing.
How selective breeding works
People choose organisms with a desired characteristic and breed them together. They then choose the offspring that show that characteristic most strongly. Repeating this over many generations gradually shifts the population towards the desired characteristic.
Examples in animals and crops
Selective breeding has produced many familiar animals and crop plants. The characteristics chosen depend on what people want from each organism. Dairy cattle are selected for milk yield. Modern dairy cows produce much more milk than their ancestors. Beef cattle and sheep are selected for meat yield and growth rate, so they reach slaughter weight faster. Wheat and other cereals are selected for larger grains, disease resistance and shorter stalks that are less likely to fall over in wind. Domestic dogs were bred from wolves. They have been selected for size, shape, temperament…
Why selective breeding is not genetic modification
Selective breeding does not add a gene. It uses alleles, which are different forms of a gene, that already exist in the population. By choosing which organisms reproduce, people make the alleles linked to a desired characteristic more common over generations. For example, farmers breed together cows that already produce the most milk. This makes alleles for high milk yield more common in later generations.
Why selective breeding can cause problems
Only a few individuals may be chosen to reproduce, so fewer different alleles are passed on. This reduces genetic variation in the population. If closely related organisms are bred together, harmful recessive alleles are more likely to be inherited in pairs. When an individual has two copies of the same allele, it is homozygous, so a harmful recessive characteristic is more likely to appear. Repeated selection can make the population less genetically varied. Inbreeding increases the chance that offspring inherit two copies of a harmful recessive allele.