4.2Unit 2 · Use of Biological Resources
Selective Breeding — Unit 2 — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions
5 min read 4 parts Lesson 3 of 5 1 quiz
Lesson summary
Define selective breeding.Selective breeding works because the chosen characteristics are controlled by genes that are inherited.It takes many generations, so it is slow, especially for animals with long life cycles.Modern bread wheat was bred for high yield and short stems.Holst…
- Definition
- Plant method
- Animal method
- Natural vs artificial selection
- Examples of selective breeding
- Describe how a farmer could produce a herd of cattle with high milk yield
Section 1 of 4Selective Breeding — Unit 2
Definition:
selective breeding (artificial selection) is when humans choose parents with desired characteristics and breed them, then choose the best offspring to breed, repeating for many generations.Plants:
wheat and rice have been bred for high yield, short stems (so energy goes into grain, and plants are not flattened by wind and rain), disease resistance, tolerance of drought or frost, and uniform ripening for machine harvesting.Plant method:
choose two varieties with different useful traits (e.g. one high-yielding, one disease-resistant) and cross-pollinate them by transferring pollen with a brush, covering flowers to stop other pollen.- Grow the seeds, select plants with both traits and repeat.
Animals:
cattle are bred for high milk yield or rapid growth and meat quality; chickens for more eggs or faster growth; sheep for wool; animals may also be bred to be docile and easier to handle.Animal method:
pick the male and female with the best characteristics (e.g. a cow with high milk yield and a bull whose mother had high milk yield), breed them, select the best offspring and continue over generations.- Artificial insemination can spread a good bull's genes widely.
Problems:
the gene pool is reduced, so a population may lack alleles to resist a new disease or environmental change.- Inbreeding can bring harmful recessive alleles together, causing health problems.
- Some bred animals may suffer, e.g. very fast-growing chickens can have leg problems.
Natural vs artificial selection:
Natural selection happens without humans and produces features that help survival in the environment.- Artificial selection only happens when humans choose which organisms breed, producing features useful to humans (not necessarily to the organism), and is usually quicker.
Examples of selective breeding:
Wild brassica has been bred into cabbage, cauliflower, broccoli, kale and Brussels sprouts. All dog breeds descend from wolves, selected for traits such as a gentle nature or speed.
Section 2 of 4Process & sequence
- Decide which characteristic is wanted.
- Choose parents that best show the characteristic.
- Breed them together.
- Select the offspring that best show the characteristic.
- Breed these together.
- Repeat over many generations until all offspring show the characteristic reliably.
Selective breeding cycle — write these steps in this order; each step is a marking point.
- Choose desired trait
- Select best parents
- Breed them
- Select best offspring
- Repeat for many generations
Section 3 of 4Compare
| Organism | Desired characteristic | Benefit |
|---|---|---|
| Wheat | High yield, short stems | More grain; less damage from wind |
| Crop plants | Disease resistance | Fewer losses, less pesticide |
| Dairy cattle | High milk yield | More milk per cow |
| Beef cattle | Fast growth, muscle mass | More meat |
| Sheep | Fine, thick wool | Better textile quality |
| Advantages | Disadvantages |
|---|---|
| Increases yield of food | Reduces genetic variation |
| Can produce disease-resistant crops | Population vulnerable to new diseases |
| Animals can be made more docile | Inherited defects more likely (inbreeding) |
| Uses natural reproduction | Takes many generations |
Section 4 of 4Worked examples
Describe how a farmer could produce a herd of cattle with high milk yield.
- Select the cows with the highest milk yield and breed them with a bull whose female relatives produced high milk yield.
- Select the female offspring with the highest milk yield and breed them again. Repeat over many generations until the whole herd reliably produces a high yield.
- Select the cows with the highest milk yield and breed them with a bull whose female relatives produced high milk yield.
Explain why a crop produced by selective breeding may be badly affected by a new disease.
- Selective breeding reduces genetic variation, so the plants are very similar.
- If none carry an allele for resistance to the new pathogen, all plants are susceptible and the whole crop could be lost.
- Selective breeding reduces genetic variation, so the plants are very similar.
Marked practiceQuizzes for this lesson
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