Selective breeding: Animals which produce more meat or milk.

The idea

Selective breeding is the deliberate mating of organisms with desirable characteristics so that those features become more common in later generations. For farm animals, farmers choose parents that already yield more meat or milk than average. Their offspring inherit the alleles linked to high yield, and the best offspring are bred again over many generations. For example, a dairy farmer may record milk yield for each cow, keep the daughters of the highest-yielding cows, and repeat the process. Meat production can be improved similarly by breeding from animals with greater muscle mass. Selection reduces the frequency of less desirable alleles in the population, but it can also reduce genetic variation and make a breed more vulnerable to disease or environmental change.

What earns the marks

• Selective breeding involves humans choosing which organisms reproduce, rather than allowing random mating.
• Farmers identify and breed from animals that already show high meat yield or high milk yield.
• The offspring inherit alleles associated with the desirable characteristic, so the characteristic becomes more common in later generations.
• Selection is repeated over many generations, gradually increasing the proportion of animals with high yield.
• Records such as milk volume per cow or muscle mass can be used to decide which animals to breed.
• Selective breeding reduces genetic variation because only a small number of parents contribute to each generation.

Common mistakes and how to put them right

• Thinking that an individual animal can be changed by selective breeding during its own lifetime.

  Put it right: Selective breeding changes the inherited characteristics of future generations, not the chosen parent.

• Confusing selective breeding with genetic engineering.

  Put it right: Selective breeding selects existing alleles through reproduction, whereas genetic engineering transfers genes directly.

• Assuming that only one generation of selection is needed.

  Put it right: The improvement is gradual and usually requires repeated selection over many generations.

Exam tips

• Name the specific characteristic being selected, such as milk yield or meat yield, rather than writing only that the animals are 'better'.
• Use the word 'inherit' to link the chosen parents to the offspring and explain why the desirable allele becomes more common.
• If asked to evaluate the process, mention both the benefit of higher yield and the risk from reduced genetic variation.