Selective breeding — AQA GCSE Combined Science
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 desirable characteristics and breed them together, repeatedly over many generations.
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For food plants, farmers may select wheat with high yield or disease resistance; for domesticated animals, they may select cows that produce more milk or chickens that grow quickly. The impact is that the chosen features become more common in the population, so food production can increase. However, selective breeding reduces genetic variation, because only a few individuals are bred. This makes populations more vulnerable to new diseases or changes in the environment, and can cause health problems in some domesticated animals.
Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics. Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals.
Selective breeding, also called artificial selection, is a human-driven process in which people choose which plants and animals reproduce so that particular genetic characteristics become more common in later generations. Unlike natural selection, the selection pressure is applied by humans rather than by the environment. The chosen parents pass on alleles for the desired features to their offspring, so those features appear more often over time. This has been practised for thousands of years: early farmers saved seed from the best wild plants to grow food crops, and people bred the tamest, most useful domesticated animals. For example, a farmer might breed only cows that produce the most milk, or only wheat plants that give the highest yield. The process changes the allele frequencies in the population and can eventually produce new breeds or varieties.
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.
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.
The characteristic can be chosen for usefulness or appearance:
Selective breeding works because variation exists within a species, and humans decide which variants become parents of the next generation. A characteristic is chosen either for usefulness, such as higher milk yield in cattle, disease resistance in wheat or larger fruit size, or for appearance, such as flower colour in roses, feather colour in poultry or coat colour in dogs. The chosen parents are bred, and their offspring that show the desired characteristic most strongly are bred again. Repeating this over many generations increases the frequency of the chosen alleles in the population. The characteristic must be heritable, so selecting a feature caused only by environment, such as a plant grown with extra fertiliser, will not reliably pass it on.
Disease resistance in food crops.
Disease resistance in food crops is a useful characteristic that breeders select for because resistant plants survive infection and produce a more reliable yield. Within a crop species, some individual plants naturally carry alleles giving resistance to a particular fungus, bacterium or virus. Breeders identify these plants, cross them with high-yielding plants, and then select offspring that show both resistance and good yield. This process is repeated over several generations. The result is a crop population in which resistance alleles are more common, so fewer plants are lost to disease and farmers may need fewer chemical treatments. Resistance is only effective against the specific pathogen concerned, and a new pathogen or a change in the pathogen population can overcome it.
Animals which produce more meat or milk.
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.
Domestic dogs with a gentle nature.
Selective breeding can also target behaviour. Dog breeders may choose parents that show a gentle, calm temperament, then breed them together. Puppies that inherit alleles contributing to that temperament are more likely to be gentle, and the gentlest offspring may be bred in the next generation. Over many generations, gentle behaviour becomes more common in that line of dogs. Behaviour is influenced by many genes and by environment, training and handling, so selection is rarely as clear-cut as selecting for a single visible feature. Breeders often observe adult dogs in different situations before deciding which to breed. Selection for temperament can reduce genetic variation and may unintentionally increase inherited health problems if closely related dogs are used.
Large or unusual flowers.
Selective breeding (artificial selection) is used by humans to develop organisms with desirable characteristics. For flowering plants, growers choose parents that already show the wanted feature, such as large blooms or unusual petal shapes and colours, then breed them together. Their offspring vary, so only those showing the desired flowers most strongly are kept and bred again. This is repeated over many generations, so the alleles controlling large or unusual flowers become more common in the population. For example, a plant with wide petals is crossed with another wide-petalled plant; seedlings with narrow petals are discarded, while the widest-flowered plants are crossed. The characteristic is inherited, so the change is genetic, not simply caused by better feeding or care.
Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects.
Selective breeding reduces genetic variation because a small number of chosen organisms are repeatedly bred together. When closely related individuals breed, this is inbreeding. Inbred offspring are more likely to inherit two copies of the same harmful recessive allele, so inherited defects become more common. Breeds may also share similar alleles that give poor resistance to disease, so a single pathogen can affect many individuals. For example, some pedigree dog breeds are prone to hip problems or breathing difficulties, and some crop varieties are vulnerable to a new disease. Breeders can reduce the risk by introducing unrelated individuals to increase genetic variation, although this may reduce uniformity of the desired characteristic.
Your focus
- Describe how selective breeding is carried out in food plants and domesticated animals.
- Explain the benefits of selective breeding for food production and human use.
- Explain the risks of selective breeding, including reduced genetic variation and inherited health problems.
Show all 27 objectives
- Define selective breeding and state that it is also called artificial selection.
- Describe how humans choose parents with desired characteristics and breed them together.
- Explain that selective breeding has been used for thousands of years to produce food crops and domesticated animals.
- Describe the steps of selective breeding from choosing parents to breeding selected offspring.
- Explain why selection must continue over many generations to change the population.
- State that the final result is a population in which all offspring show the desired characteristic.
- State that a characteristic may be selected for usefulness or for appearance.
- Describe the process of selecting and breeding parents over several generations.
- Explain why only heritable characteristics can be changed by selective breeding.
- State that disease resistance in food crops is a useful characteristic selected by breeders.
- Describe how resistant plants are crossed and selected over several generations.
- Explain the benefit of disease resistance for crop yield and the limitation that resistance is pathogen-specific.
- Describe how farmers use selective breeding to increase meat or milk yield in animals.
- Explain why repeated selection over generations changes the frequency of desirable alleles in a population.
- Evaluate one benefit and one risk of selective breeding for high meat or milk production.
- Describe how dog breeders can select for a gentle nature over several generations.
- Explain why inherited alleles and environmental factors both affect a dog's temperament.
- Evaluate a welfare or genetic-variation concern arising from selective breeding for temperament.
- Describe how plants with large or unusual flowers can be produced by selective breeding.
- Explain why offspring vary and why only some are chosen to breed.
- Relate the repeated selection of flowering plants to an increase in the frequency of the desired alleles.
- Explain how selective breeding can lead to inbreeding.
- Describe why inbreeding increases the risk of inherited defects and disease in a breed.
- Suggest how introducing unrelated individuals can reduce inbreeding problems.
Selective breeding exam tips
Marking Points
- Selective breeding involves choosing parents with desired characteristics and breeding them together over several generations.
- In food plants, examples include selecting for higher yield, disease resistance or better flavour, which can improve food security.
- In domesticated animals, examples include selecting for increased milk or meat production, or for temperament, which can benefit farmers.
- A negative impact is reduced genetic variation, making populations more susceptible to disease or environmental change.
- Another negative impact can be inherited health problems in some domesticated animals, such as breathing difficulties in certain breeds.
- Selective breeding is also known as artificial selection because humans, not the environment, decide which organisms reproduce.
- Humans choose parents that already show a desired genetic characteristic, such as high milk yield in cattle or large grain size in wheat.
- The desired characteristic is controlled by alleles, so selected parents pass those alleles to their offspring.
- Over many generations the proportion of individuals showing the desired characteristic increases in the population.
- The process has been used for thousands of years, beginning with the domestication of wild plants and animals for food.
- Selective breeding differs from natural selection because the selection pressure is applied by humans rather than by environmental factors.
- 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.
- Selective breeding requires inherited variation in the chosen characteristic within the population.
- Usefulness examples include higher yield, disease resistance, hardiness or improved nutritional content.
- Appearance examples include colour, size, shape or other physical features valued by breeders.
- The chosen parents are bred together and the best offspring are selected to breed in the next generation.
- Repeating selection over many generations increases the proportion of individuals showing the desired characteristic.
- Only heritable characteristics respond to selection; environmentally caused features do not reliably pass to offspring.
- Disease resistance is a heritable characteristic that can be selected for in food crops.
- Resistant plants survive disease better and give a more reliable yield for farmers.
- Breeders cross resistant plants with plants showing other desirable features such as high yield.
- Offspring showing resistance are selected and bred over several generations to increase resistance allele frequency.
- Reduced disease loss can lower the need for chemical control measures.
- Resistance is specific to particular pathogens and may be overcome if the pathogen changes.
- 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.
- Humans choose dogs with a gentle nature as parents, rather than allowing random mating.
- Gentle behaviour is influenced by inherited alleles as well as by environment and training.
- Offspring of gentle parents are more likely to inherit alleles associated with a gentle temperament.
- Breeders observe and assess adult dogs before selecting which ones to breed.
- Repeated selection over many generations makes gentle behaviour more common in that population.
- Selective breeding for temperament can reduce genetic variation and increase the risk of inherited health problems.
- Selective breeding involves humans choosing which organisms reproduce, rather than leaving reproduction to natural selection.
- Parents are selected because they already show the desired characteristic, for example large flowers or unusual petal colour, shape or number.
- The selected parents are bred together and their offspring show variation in flower size or appearance.
- Only offspring with the most desirable flowers are allowed to breed in the next generation.
- This process is repeated over many generations, increasing the proportion of alleles for large or unusual flowers in the population.
- The change is inherited because the selected alleles are passed on in gametes, so it is a genetic change rather than an environmental effect.
- Selective breeding often uses a small number of closely related parents, which reduces genetic variation in the breed.
- Inbreeding means breeding between closely related individuals, so offspring are more likely to inherit identical alleles from both parents.
- Harmful recessive alleles are more likely to be inherited in pairs, increasing the chance of inherited defects appearing.
- Reduced genetic variation means a breed may be less able to resist new diseases or environmental changes.
- A disease or inherited defect can become particularly common in a breed because many individuals share the same susceptible alleles, meaning they lack resistance rather than possess resistance.
- Introducing unrelated organisms can increase genetic variation and reduce the risk of inbreeding problems.
Examiner Tips
- 💡Give at least one named example for a food plant and one for a domesticated animal to show understanding of both contexts.
- 💡Balance your answer by explaining both benefits, such as increased yield, and drawbacks, such as reduced genetic variation.
- 💡Use the term 'genetic variation' when explaining why selectively bred populations may be at greater risk from disease.
- 💡Define selective breeding in one clear sentence, then give a named example such as dairy cattle or wheat to show understanding.
- 💡Use the phrase 'desired characteristic' and explain that humans choose the parents, so the selection is artificial rather than natural.
- 💡When comparing with natural selection, state that the selection pressure comes from humans in selective breeding but from the environment in natural selection.
- 💡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.
- 💡Name the organism, the chosen characteristic and whether the reason is usefulness or appearance in your first sentence.
- 💡Use the phrase heritable variation to show the characteristic can be passed on.
- 💡Describe the cycle of selection and breeding rather than only stating the final outcome.
- 💡Link disease resistance to food security or reliable crop yield to show why it is useful.
- 💡Use the term pathogen when naming the organism that causes the disease.
- 💡Describe selection across generations rather than implying resistance appears immediately in one cross.
- 💡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.
- 💡State clearly that the breeder chooses which dogs mate, and link that choice to the gentle nature of the parents.
- 💡Use the idea of alleles being inherited when explaining why puppies may also be gentle.
- 💡When evaluating the practice, refer to reduced genetic variation or welfare concerns rather than making a general statement that it is 'bad'.
- 💡Name the characteristic and state the selection criterion, for example 'plants with the largest flowers are chosen to breed'.
- 💡Use the phrase 'over many generations' to show that the allele frequency changes gradually.
- 💡Link the desired flower feature to alleles being passed on in gametes, rather than describing only the appearance of the plant.
- 💡Define inbreeding clearly as breeding between closely related individuals.
- 💡Explain the link between reduced genetic variation and a greater chance of harmful recessive alleles being inherited.
- 💡Use a named example, such as a pedigree dog breed or a crop variety, to show how a defect or disease becomes common.
- 💡When discussing disease, state that shared susceptibility or lack of resistance alleles allows one pathogen to affect many individuals.
Common Mistakes
- Thinking selective breeding creates new alleles; correction: it selects from existing variation, so no new alleles are produced.
- Assuming selective breeding always benefits the organism; correction: it benefits humans, but can harm the organism's health or reduce its genetic diversity.
- Confusing selective breeding with genetic engineering; correction: selective breeding involves breeding existing organisms, whereas genetic engineering directly alters DNA.
- Thinking that selective breeding creates new alleles: it does not; it changes the frequency of existing alleles in the population. Correction: mutation is the source of new alleles, while selective breeding sorts existing variation.
- Confusing selective breeding with genetic engineering: selective breeding only involves choosing which organisms mate, whereas genetic engineering transfers genes directly. Correction: state clearly that no direct gene transfer occurs in selective breeding.
- Believing that selective breeding is a modern technology: it has been used for thousands of years since the first food crops and domesticated animals. Correction: link the process to early farming and domestication.
- Breeding all offspring rather than only those with the desired characteristic: this slows or prevents the change. Correction: select only the offspring that show the desired feature each generation.
- Stopping after one generation and expecting all offspring to show the characteristic: variation means this is unlikely. Correction: state that many generations of selection are needed.
- Assuming the desired characteristic is guaranteed in every offspring immediately: inheritance involves chance, so selection must continue over generations. Correction: describe the gradual increase in the proportion of desired offspring.
- Thinking any chosen feature will be inherited: correct by checking that the characteristic is controlled by genes and shows variation between individuals.
- Confusing selective breeding with genetic engineering: correct by stating that selective breeding crosses existing organisms and does not directly transfer genes between species.
- Assuming one round of breeding is enough: correct by explaining that many generations of selection are usually needed to change the population.
- Classifying temperament as an appearance characteristic; correction: temperament is behavioural, not physical appearance, so it is not an appearance example.
- Saying resistant plants never get any disease: correct by explaining resistance reduces the effect of a specific pathogen rather than giving total immunity to all diseases.
- Ignoring yield while focusing only on resistance: correct by noting that breeders usually combine resistance with high yield and other useful features.
- Treating resistance as a single permanent feature: correct by stating that pathogen populations can change and overcome resistance, so breeding is ongoing.
- Thinking that an individual animal can be changed by selective breeding during its own lifetime; correction: selective breeding changes the inherited characteristics of future generations, not the chosen parent.
- Confusing selective breeding with genetic engineering; correction: selective breeding selects existing alleles through reproduction, whereas genetic engineering transfers genes directly.
- Assuming that only one generation of selection is needed; correction: the improvement is gradual and usually requires repeated selection over many generations.
- Claiming that temperament is entirely genetic; correction: behaviour is affected by both inherited alleles and the environment, including training and handling.
- Thinking that a gentle dog will automatically produce only gentle puppies; correction: inheritance involves chance, so not every puppy inherits the same combination of alleles.
- Confusing selection for behaviour with training; correction: training changes an individual dog's behaviour, while selective breeding changes the inherited characteristics of future generations.
- Thinking the plant grows larger flowers simply because it is given more water or fertiliser; correction: selective breeding changes allele frequencies, so the improvement is inherited.
- Believing that all offspring of two large-flowered parents will automatically have large flowers; correction: offspring vary, so selection must continue over several generations.
- Confusing selective breeding with genetic engineering; correction: selective breeding crosses existing organisms and selects offspring, whereas genetic engineering transfers a specific gene.
- Saying inbreeding creates new harmful mutations; correction: it increases the chance that existing harmful recessive alleles are inherited from both parents.
- Assuming selective breeding always improves health; correction: it can increase the risk of disease or inherited defects when closely related organisms are bred.
- Confusing inbreeding with natural selection; correction: inbreeding is a consequence of human-controlled breeding of related individuals, not selection by the environment.
- Saying shared alleles for resistance make a breed vulnerable; correction: vulnerability arises when many individuals share alleles that give poor resistance or susceptibility to a disease.