AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Selective breeding: 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.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- 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.
Common mistakes and how to put them right
Saying inbreeding creates new harmful mutations.
Put it right: It increases the chance that existing harmful recessive alleles are inherited from both parents.
Assuming selective breeding always improves health.
Put it right: It can increase the risk of disease or inherited defects when closely related organisms are bred.
Confusing inbreeding with natural selection.
Put it right: 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.
Put it right: Vulnerability arises when many individuals share alleles that give poor resistance or susceptibility to a disease.
Exam tips
- 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.
Try the lesson
The start of the Selective breeding: Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects. lesson, made from checked teaching notes for this specification point.
Have a go first
What is meant by inbreeding in the context of selective breeding?
Listen
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. A strong answer earns marks for these points. 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 common slip: Saying inbreeding creates new harmful mutations. Instead, it increases the chance that existing harmful recessive alleles are inherited from both parents. Exam tip: Define inbreeding clearly as breeding between closely related individuals.
Key words
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Inbreeding
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Reduced genetic variation
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Inheritance of harmful recessive alleles
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Susceptibility to disease
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See every definition
- Inbreeding
- Breeding between closely related individuals, which happens when a small number of chosen organisms are repeatedly bred together.
- Reduced genetic variation
- A result of selective breeding using a small number of closely related parents, meaning a breed may be less able to resist new diseases or environmental changes.
- Inheritance of harmful recessive alleles
- Inbred offspring are more likely to inherit two copies of the same harmful recessive allele, so inherited defects become more common.
- Susceptibility to disease
- Breeds may share similar alleles that give poor resistance to disease, so a single pathogen can affect many individuals.
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