5b Selective Breeding¶
Part of 5 Use of Biological Resources.
In selective breeding, people choose which organisms breed, so the alleles for a wanted characteristic become more common in each generation. It works quickly and has been used for thousands of years, but every round of selection also narrows the variety of alleles in the population, and that is its cost.
What You Need to Learn¶
Further detail: Pearson Edexcel International GCSE Biology specification.
How selective breeding works, which characteristics are chosen in plants and animals, and the genetic risks of reducing variation.
How Selective Breeding Works¶
Selective breeding, also called artificial selection, is the process in which humans choose which organisms breed, to produce offspring with a desired characteristic. It has been practised since animals were first domesticated and plants first grown for food. The steps are:
- Choose parents that show the desired characteristic most strongly.
- Breed them together.
- From the offspring, select those that show the characteristic most strongly and breed them together.
- Repeat for many generations.
Over time the alleles for the characteristic become more frequent in the population, and the characteristic becomes more pronounced. This is the same change in allele frequency as in natural selection, but people do the selecting.
| Natural selection | Selective breeding | |
|---|---|---|
| What selects | The environment, through survival and reproduction | People, by choosing the parents |
| What is favoured | Features that help survival | Features that people want |
| Result | Adaptation to the environment | A population with the chosen characteristic |
Worked example: selecting for milk yield
Suppose the average yield of a herd is 20 litres per day. The farmer breeds only from the cows and bulls whose mothers gave the highest yields. The next generation averages 22 litres, and breeding again from the highest yielders gives 24 litres.
Each generation, the alleles linked to a high yield are present in more of the breeding animals, so the average rises. The gain is steady only while enough variation remains to select from.
Uses in Plants and Animals¶
Selective breeding can be used for any inherited characteristic:
| Organism | Characteristics selected |
|---|---|
| Plants | Larger fruits, higher grain yields, disease resistance, drought tolerance, more vigorous growth |
| Animals | Higher meat yield, greater milk production, disease resistance, egg production, particular coat or behaviour |
Limits of Selective Breeding¶
Selective breeding concentrates some alleles, and in doing so it removes others. This creates three linked risks:
- A reduced gene pool. Selecting for the same alleles means breeding closely related individuals, so the variety of alleles in the population falls.
- Inbreeding. Closely related individuals are more likely to carry the same recessive alleles, so offspring are more likely to be homozygous for them and to show harmful recessive conditions.
- Vulnerability. A population with little variation has a poorer chance that some individuals carry an allele that gives resistance to a new disease or that suits a changed environment. A new pathogen could then affect the whole population.
Worked example: why inbreeding is risky
Two closely related dogs are both carriers of a harmful recessive allele (Aa × Aa). The Punnett square gives AA, Aa, Aa and aa, so each puppy has a 1 in 4 chance of being aa and showing the condition. Unrelated dogs are much less likely to both carry the same rare allele, so the chance is far lower.
Exam technique
For a question on the disadvantages of selective breeding, link each point to the loss of variation: fewer alleles in the gene pool, so more inbreeding and more harmful recessive conditions, and less chance that any individual resists a new disease. "It is not natural" is not a biological reason.
Common Confusions¶
- Selective breeding vs genetic modification: Selective breeding works within a species, using alleles that already exist. Genetic modification can introduce genes from other species.
- Selective breeding vs natural selection: In both, allele frequencies change, but in selective breeding the people choose the parents.
- Short-term gain vs long-term risk: Selective breeding can improve a trait quickly while creating genetic weakness if variation is reduced too far.
Check Yourself¶
- Describe the steps of selective breeding to produce wheat with a higher grain yield.
- State one difference between natural selection and selective breeding.
- Explain why the alleles for a desired characteristic become more common in a selectively bred population.
- Explain why a herd of selectively bred cattle is more at risk from a new disease than a mixed wild herd.
- Explain why inbreeding increases the chance of a harmful recessive condition.
Answers
- Choose the wheat plants with the highest yield and breed them together. Select the offspring with the highest yield and breed those. Repeat over many generations.
- In natural selection the environment selects through survival and reproduction, but in selective breeding people choose which organisms breed.
- Only the organisms with the characteristic are allowed to breed, so their alleles are passed on to the next generation, and the alleles become more frequent each generation.
- The cattle are closely related and have little genetic variation, so it is less likely that any of them carries an allele giving resistance. A wild herd has more variation, so some individuals are more likely to survive.
- Closely related individuals are more likely to carry the same recessive allele, so offspring are more likely to inherit two copies and be homozygous recessive.
Key Terms¶
- Selective breeding: the process of choosing organisms with desired characteristics and breeding them over many generations to increase the frequency of that characteristic.
- Desired characteristic: a heritable trait that breeders wish to increase in a population.
- Inbreeding: breeding between closely related individuals, which reduces genetic diversity.
- Gene pool: the complete set of all alleles present in a population.
- Artificial selection: another term for selective breeding — human-directed selection for particular traits.