Selective breeding — AQA GCSE Biology
Test yourself on Selective breeding with AQA GCSE practice questions.
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Selective breeding explained
Selective breeding is when humans choose organisms with desired characteristics and breed them together.
Read the full explanation
Over many generations the useful alleles become more common, so food plants may give higher yields, better disease resistance or improved taste, and domesticated animals may produce more milk, meat or wool or have a calmer temperament. A useful method is to identify the best individuals, breed them, then repeat with their best offspring. The impact includes reliable food supplies and improved productivity, but also reduced genetic variation, which makes populations vulnerable to new diseases, and possible welfare problems in animals bred for extreme features.
Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics.
Selective breeding, also called artificial selection, is a human-directed process that changes the inherited characteristics of a population over generations. Humans decide which individuals become parents, so alleles for chosen traits become more common while unwanted alleles become rarer. For example, a farmer wanting higher milk yield measures yield in a herd, breeds the highest-yielding cows with a high-yielding bull, then repeats the selection with the best offspring. The process relies on natural genetic variation and inheritance; it does not create new alleles. It differs from natural selection because humans, not environmental pressures, determine which individuals reproduce. The chosen characteristic must be heritable, and selection usually continues over many generations before a clear population-level change appears.
Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals.
Selective breeding is not a modern technique; it began thousands of years ago when humans first domesticated wild plants and animals. Early farmers saved seed from the best food crops and bred the most useful animals, gradually producing domesticated varieties. For example, wild grasses were selected over many generations to become cereal crops with larger grains and stronger stems, while wild cattle were selected for docility and milk production. This long history explains why many farmed species differ greatly from their wild ancestors. The underlying process is the same as modern selective breeding: humans choose which individuals reproduce, so desired inherited characteristics become more common in later generations.
Selective breeding involves choosing parents with the desired characteristic from a mixed population.
In a mixed population, individuals vary in their characteristics because they have different combinations of alleles. Selective breeding starts by identifying the individuals that show the desired characteristic, such as high milk yield in cattle or large fruit size in tomato plants. These individuals are chosen as parents, while those without the desired characteristic are not allowed to breed. Choosing from a mixed population is essential because the variation provides the raw material for selection. For example, a farmer measures milk yield in a herd, records the highest producers, and selects those cows and a high-yielding bull for breeding. The chosen parents must be able to pass the characteristic to their offspring.
They are bred together.
Once parents with the desired characteristic have been chosen from a mixed population, they are bred together. Their offspring inherit alleles from both parents, so some offspring may show the desired characteristic. The best offspring are then selected and bred together in the next generation. This cycle of selecting parents and breeding them is repeated over many generations. For example, a farmer breeds a high-yielding cow with a high-yielding bull, then selects the highest-yielding offspring to breed in the next round. Over time, the proportion of individuals with the desired characteristic increases. Breeding together selected parents is therefore the step that allows human choice to change the population.
From the offspring those with the desired characteristic are bred together.
Once the first generation of offspring has been produced, the breeder examines the young and identifies which individuals actually display the desired characteristic. Only those individuals are kept and allowed to breed with each other; the rest are removed from the breeding programme. This is the selection step, and it is repeated each generation. For example, a farmer wanting disease-resistant wheat grows plants from crossed parents, then breeds together only the seedlings that survive infection. Because the chosen parents both carry the favourable alleles, a greater proportion of their offspring should show the characteristic, so the population mean shifts towards the desired trait over successive rounds.
This continues over many generations until all the offspring show the desired characteristic.
Selective breeding is not a single event; it is a repeated cycle. In each generation the breeder selects individuals showing the desired characteristic and breeds them together. Because those parents are more likely to carry the favourable alleles, more of their offspring show the trait. Repeating the cycle over many generations steadily raises the frequency of the favourable alleles in the population. Eventually, if selection is strong and the trait is reliably inherited, nearly all offspring show the desired characteristic, producing a breed or variety that breeds true for that trait. In practice, complete uniformity may take many years and can be limited by genetic variation, environmental effects and the appearance of inherited problems.
The characteristic can be chosen for usefulness or appearance: • Disease resistance in food crops. • Animals which produce more meat or milk. • Domestic dogs with a gentle nature. • Large or unusual flowers.
Humans choose which characteristic to select for, based on usefulness, appearance, or behaviour. Usefulness includes traits improving food production, such as disease resistance in crops or higher meat or milk yield in farm animals. Appearance includes traits valued for aesthetics, such as large or unusual flowers in ornamental plants. Behavioural traits are also selected, such as a gentle nature in domestic dogs for companionship. The same underlying process applies in every case: identify individuals showing the desired trait and breed them together. Selective breeding is driven by human priorities, which may be economic, agricultural, aesthetic, or behavioural, rather than by natural selection.
Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects.
Repeatedly breeding together individuals that show the same desired characteristic reduces genetic variation in the population. If closely related individuals are bred, the population becomes inbred. Inbreeding increases the chance that harmful recessive alleles are inherited from both parents, so offspring may show inherited defects. It also reduces the ability of the population to adapt to new diseases or environmental changes, so some breeds become particularly prone to disease. Examples include certain dog breeds with inherited breathing or joint problems and some crop varieties that are vulnerable to a new pathogen. Breeders may reduce the risk by introducing unrelated individuals to restore variation.
Your focus
- Describe how humans selectively breed food plants and domesticated animals.
- Explain the benefits of selective breeding for food production.
- Explain at least one drawback of selective breeding, such as reduced genetic variation.
Show all 27 objectives
- Define selective breeding and state that it is also called artificial selection.
- Explain how humans choose parents with a desired heritable characteristic.
- Distinguish selective breeding from natural selection by identifying the agent of selection.
- State that selective breeding has been carried out for thousands of years.
- Give an example of a food crop bred from a wild plant or an animal domesticated by selective breeding.
- Explain that early farmers selected visible characteristics without knowledge of genes.
- Identify the desired characteristic in a given breeding scenario.
- Explain why a mixed population is needed for selective breeding.
- State that chosen parents must be able to pass the desired characteristic to their offspring.
- Describe the breeding step in the selective breeding process.
- Explain that offspring inherit alleles from both selected parents.
- Outline the repeated cycle of selection and breeding over many generations.
- Identify which offspring show the desired characteristic after a cross.
- Describe how selected offspring are bred together to produce the next generation.
- Explain why breeding selected individuals increases the proportion of offspring with the desired characteristic.
- Describe the repeated cycle of selection and breeding over many generations.
- Explain how allele frequencies change across generations of selective breeding.
- State the endpoint of selective breeding as a population in which all offspring show the desired characteristic.
- Give examples of characteristics selected for usefulness in food crops and farm animals.
- Give examples of characteristics selected for appearance or temperament in plants and dogs.
- Explain that the characteristic selected for is chosen by humans for usefulness, appearance or behaviour.
- Define inbreeding as breeding together closely related individuals.
- Explain how inbreeding increases the chance of inherited defects and disease susceptibility.
- Suggest how breeders can reduce the effects of inbreeding by increasing genetic variation.
Selective breeding exam tips
Marking Points
- States that humans choose organisms with desired characteristics to breed together.
- Explains that useful alleles become more common over many generations.
- Gives examples of desired characteristics in food plants, such as yield, disease resistance or taste.
- Gives examples of desired characteristics in domesticated animals, such as milk yield, meat yield or temperament.
- Explains a negative impact such as reduced genetic variation or welfare problems.
- Selective breeding is also known as artificial selection because humans choose which organisms reproduce.
- Humans select parents that already show the desired genetic characteristic, such as high milk yield or large fruit size.
- The desired characteristic must be inherited, so selected parents pass alleles for it to their offspring.
- Repeating selection over several generations increases the proportion of individuals showing the desired characteristic.
- Selective breeding acts on existing genetic variation and does not create new alleles.
- It differs from natural selection because the selection pressure is applied by humans rather than by the environment.
- Selective breeding has been practised by humans for thousands of years.
- Early farmers bred food crops from wild plants by saving and planting seed from preferred plants.
- Wild animals were domesticated by breeding individuals with useful characteristics such as docility or milk yield.
- Domestication produced populations that differ from their wild ancestors.
- The same principle of human selection has been used from early farming to modern agriculture.
- A mixed population contains individuals with a range of characteristics due to genetic variation.
- Humans identify and choose individuals that show the desired characteristic.
- Individuals without the desired characteristic are not selected as parents.
- The desired characteristic must be heritable so that it can be passed to offspring.
- Selection from a mixed population relies on existing genetic variation rather than creating new alleles.
- Selected parents are mated or crossed so that they reproduce together.
- Offspring inherit alleles from both selected parents.
- Some offspring may show the desired characteristic and are selected for the next generation.
- The cycle of selecting parents and breeding them is repeated over many generations.
- Repeated breeding of selected individuals increases the proportion of the population with the desired characteristic.
- The breeder inspects the offspring of the initial cross and identifies individuals that show the desired characteristic.
- Only offspring showing the desired characteristic are selected as parents for the next generation.
- Selected individuals are bred together, not with unselected individuals from the wider population.
- Offspring lacking the desired characteristic are not used for further breeding.
- The process is repeated each generation, so the proportion of offspring with the desired characteristic tends to increase.
- Selection acts on phenotype, but the underlying effect is to increase the frequency of favourable alleles in the breeding population.
- Selective breeding is a repeated cycle carried out over many generations.
- Each generation involves selecting individuals with the desired characteristic and breeding them together.
- The frequency of favourable alleles in the population increases across generations.
- The proportion of offspring showing the desired characteristic rises over time.
- The endpoint is a population in which essentially all offspring show the desired characteristic.
- The process can take many generations and may be limited by remaining variation or inherited defects.
- The characteristic selected for can be chosen for usefulness, appearance or behaviour.
- Disease resistance in food crops is an example of a useful characteristic.
- Animals that produce more meat or milk are examples of useful characteristics.
- Domestic dogs with a gentle nature are an example of a characteristic chosen for behaviour or temperament.
- Large or unusual flowers are an example of a characteristic chosen for appearance.
- In each case the breeder selects individuals showing the desired characteristic and breeds them together.
- Selective breeding can reduce genetic variation within a breed or variety.
- Inbreeding occurs when closely related individuals are bred together.
- Inbreeding increases the chance of offspring inheriting harmful recessive alleles from both parents.
- Inbred breeds may be particularly prone to disease or inherited defects.
- Reduced genetic variation makes a population less able to adapt to new diseases or environmental changes.
- Breeders can reduce inbreeding by introducing unrelated individuals to increase genetic variation.
Examiner Tips
- 💡Name the desired characteristic and the organism when giving an example.
- 💡Include both a benefit and a drawback to show balanced understanding.
- 💡Use the phrase over many generations to show the timescale of selective breeding.
- 💡Define selective breeding in one sentence, then give a named example such as dairy cattle or wheat to show understanding.
- 💡Use the phrase 'desired characteristic' and link it to inheritance, for example by stating that parents pass alleles for the characteristic to offspring.
- 💡When comparing with natural selection, state clearly who or what does the selecting: humans in selective breeding, the environment in natural selection.
- 💡Use a historical example, such as wheat from wild grasses or dogs from wolves, to show that selective breeding is ancient.
- 💡Link the historical practice to the modern definition by stating that humans have always chosen which individuals breed.
- 💡Avoid claiming exact dates or places unless they are part of the specification; 'thousands of years' is sufficient.
- 💡Name the desired characteristic and state how it is measured or observed, for example milk yield in litres per day.
- 💡Use the phrase 'mixed population' and explain that variation means some individuals are better suited to the breeding aim.
- 💡Make clear that humans, not the environment, choose which individuals become parents.
- 💡Describe the full cycle: choose parents, breed them, select the best offspring, and repeat.
- 💡Use the term 'offspring' and state that they inherit alleles from both parents.
- 💡When explaining improvement over time, refer to the increasing proportion of individuals with the desired characteristic.
- 💡Use the phrase ‘select and breed together’ to make the selection step explicit.
- 💡Link the choice of parents to the increased chance of offspring inheriting favourable alleles.
- 💡If asked to describe the process, sequence it as: choose parents, cross them, select offspring, breed selected offspring together, repeat.
- 💡Use the phrase ‘over many generations’ to show understanding of the timescale.
- 💡Explain the endpoint as a population in which all offspring show the desired characteristic.
- 💡Link the increase in offspring showing the trait to the increased frequency of favourable alleles.
- 💡Give at least one usefulness example and one appearance or behavioural example when asked to explain the range of characteristics.
- 💡Name the specific organism and the specific characteristic, for example disease resistance in wheat or large flowers in a garden plant.
- 💡Link each example back to the idea that humans choose which individuals breed.
- 💡Use the term ‘inbreeding’ and link it to reduced genetic variation.
- 💡Explain the mechanism by referring to harmful recessive alleles being inherited from both parents.
- 💡Give a named example, such as inherited breathing problems in some dog breeds, to support the explanation.
Common Mistakes
- Confusing selective breeding with genetic engineering; correct this by noting that selective breeding chooses existing variation, while genetic engineering transfers genes directly.
- Thinking selective breeding creates new alleles; correct this by explaining it increases the frequency of existing useful alleles.
- Ignoring disadvantages; correct this by including reduced genetic variation or welfare issues.
- Thinking selective breeding creates new alleles: correct this by stating that it selects from existing variation, and new alleles arise only by mutation.
- Confusing selective breeding with genetic engineering: correct this by noting that selective breeding involves breeding existing organisms, whereas genetic engineering transfers genes directly.
- Assuming one round of breeding is enough: correct this by explaining that repeated selection over many generations is usually needed for a clear change.
- Believing selective breeding is a recent scientific invention: correct this by stating that it began thousands of years ago with early farmers.
- Thinking domesticated animals are a different species from their wild ancestors: correct this by explaining that domestication produced changed populations of the same species.
- Assuming early farmers understood genes: correct this by stating that they selected visible characteristics without knowing about alleles.
- Thinking all individuals in a population are identical: correct this by stating that a mixed population shows genetic variation.
- Selecting parents that do not show the desired characteristic: correct this by choosing only individuals that clearly display the desired trait.
- Forgetting that the characteristic must be inherited: correct this by stating that selected parents pass alleles for the trait to their offspring.
- Thinking breeding together selected parents guarantees all offspring will show the desired characteristic: correct this by stating that inheritance involves chance, so only some offspring may show it.
- Stopping after one generation: correct this by explaining that the selection and breeding cycle must be repeated over many generations.
- Breeding selected individuals with unselected ones: correct this by stating that only individuals with the desired characteristic should be bred together.
- Thinking that all offspring from the first cross are automatically bred together; the correction is that only those showing the desired characteristic are selected.
- Confusing selection with genetic modification; the correction is that selective breeding chooses existing variation rather than inserting a gene.
- Assuming the desired characteristic appears in every offspring immediately; the correction is that it becomes more common over successive generations of selection.
- Believing that all offspring show the desired characteristic after one generation; the correction is that many generations of selection are usually needed.
- Thinking that selection changes an individual’s genes during its lifetime; the correction is that selection changes which alleles are passed on to the next generation.
- Assuming the process always produces a perfect result; the correction is that inbreeding and remaining variation can prevent complete uniformity.
- Thinking that only appearance can be selected for; the correction is that usefulness, such as disease resistance or yield, is also commonly selected for.
- Confusing selective breeding with natural selection; the correction is that humans choose the characteristic in selective breeding.
- Treating the examples as separate processes; the correction is that all four examples use the same select-and-breed method.
- Thinking that selective breeding always improves health; the correction is that it can increase the risk of inherited defects.
- Confusing inbreeding with selective breeding itself; the correction is that inbreeding is a consequence of breeding closely related selected individuals.
- Assuming inherited defects are caused by pathogens; the correction is that they result from harmful alleles passed from parents to offspring.