Selective breeding: Selective breeding involves choosing parents with the desired characteristic from a mixed population. They are bred together. From the offspring those with the desired characteristic are bred together. This continues over many generations until all the offspring show the desired characteristic.

The idea

Selective breeding follows a repeated cycle. First, identify a mixed population in which individuals vary, and choose the parents that show the desired characteristic, such as high milk yield in cattle or disease resistance in wheat. Breed those chosen parents together. Their offspring will vary, so examine them and select only those that also show the desired characteristic. Breed this selected group together. Repeat the selection and breeding each generation. Because the chosen parents carry alleles for the desired feature, those alleles become more common in the population. After many generations, the allele frequency shifts so that all or nearly all offspring show the desired characteristic. The population is then described as true-breeding for that feature.

What earns the marks

• Start with a mixed population that shows variation in the characteristic of interest.
• Choose parents that already display the desired characteristic and breed them together.
• Select offspring that show the desired characteristic and breed those together.
• Repeat the selection and breeding over many generations.
• The frequency of alleles for the desired characteristic increases in the population.
• Eventually all or nearly all offspring show the desired characteristic, producing a true-breeding population.

Common mistakes and how to put them right

• Breeding all offspring rather than only those with the desired characteristic.

  Put it right: Select only the offspring that show the desired feature each generation.

• Stopping after one generation and expecting all offspring to show the characteristic.

  Put it right: State that many generations of selection are needed.

• Assuming the desired characteristic is guaranteed in every offspring immediately.

  Put it right: Describe the gradual increase in the proportion of desired offspring.

Exam tips

• Write the process as a clear sequence: choose parents, breed, select offspring, breed selected offspring, repeat over generations.
• Use the term 'mixed population' to show that variation exists before selection begins.
• Link the final outcome to allele frequency: the desired alleles become more common until all offspring show the characteristic.