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5.2Use of Biological Resources

Selective Breeding — Edexcel Linear - OL Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 8 1 quiz

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Lesson summary

Selective Breeding in Plants and Animals — EDEXCEL LINEAR - OL notes: Define selective breeding, describe the process in plants and animals, explain the problems of inbreeding and compare artificial with natural selection.Selective breeding (artificial selection) means choosing i…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Choose parents with the desired characteristics
  • Breed them together

Part 1 of 6What selective breeding is and how it works

Selective breeding (artificial selection) means choosing individuals with desirable characteristics and breeding them together. The process does not stop there: not all offspring show the desired features, so those that do are selected and bred together again. This must be repeated over many successive generations before a "new breed" reliably shows the selected characteristics in all offspring.

1
Choose parents with the desired characteristics.
2
Breed them together.
3
From the offspring, select those with the desired characteristics.
4
Breed the selected offspring together.
5
Repeat for many generations.

In an exam answer you must mention repeating for many generations; selecting two parents, breeding them and stopping is not selective breeding.

Worked examplea farmer wants wheat with high yield and resistance to a fungal disease.

1
She grows many varieties, picks the plants with the largest grain and least infection, and cross-pollinates them.
2
Seeds from the best offspring are sown the next year and the best plants chosen again.
3
After many years nearly all the plants have both features.
4
Notice that the farmer decides which plants breed, unlike natural selection, where the environment decides which survive.

Part 2 of 6Selective breeding in plants

Humans have bred plants for many characteristics: disease resistance in food crops, increased yield, hardiness (drought tolerance), better-tasting fruit and large or unusual flowers. A famous example is wild brassica, which has been bred into cabbage, cauliflower, broccoli, Brussels sprouts, kale and kohlrabi, each by selecting different features such as large leaves, flower heads or buds.

Part 3 of 6Selective breeding in animals

The process is similar.

Selective breeding in animals
Individuals with the desired characteristics are bred, often from several unrelated parents so siblings need not be bred together; the offspring with the desired characteristics are selected and bred,
  • and this is repeated.
Examples: cows, goats and sheep for lots of milk or meat; chickens that lay large eggs; dogs with gentle natures; sheep with good wool; horses with fine features and fast pace.
The domestic dog, descended from wolves, has been bred into many breeds.

Part 4 of 6Problems with selective breeding

Selective breeding can cause inbreeding, when only the "best" closely related individuals are bred. This reduces the gene pool, meaning fewer different alleles in the population. As a result:

Problems with selective breeding
organisms are more vulnerable to new diseases, because resistant alleles may be missing;
there is a greater chance of inheriting harmful genetic defects.

Further consequences of reduced variationif a farm uses one highly bred variety, an outbreak of a new disease can destroy the whole crop, as every plant has the same weakness.

Problems with selective breeding
This is why breeders keep wild relatives and older varieties as a source of useful alleles,
  • such as drought tolerance or disease resistance, that can be bred back into modern varieties.
In animals, harmful recessive alleles become more likely to appear in homozygous form when closely related individuals mate,
  • which explains the extra health problems in some very inbred breeds.

Part 5 of 6Natural selection compared with artificial selection

Natural selectionArtificial selection
Occurs naturallyOnly occurs when humans intervene
Produces populations better adapted to their environment and survivalProduces features useful to humans, not necessarily to the survival of the individual
Usually takes a long timeTakes less time as only individuals with desired features reproduce

Part 6 of 6Worked examples

  1. Describe how a farmer could produce a variety of cattle that gives more milk. [4]4 marks

  2. Explain one problem with selective breeding. [3]3 marks

  3. State two differences between natural and artificial selection. [2]2 marks

  4. Name one characteristic selected in crop plants and give a reason. [2]2 marks

Key terms

TermMeaning
Selective breedingChoosing and breeding individuals with desired characteristics over generations.
InbreedingBreeding between closely related individuals.
Gene poolAll the alleles in a population.
Artificial selectionAnother name for selective breeding.
BreedA group within a species with selected characteristics.

Avoid these mistakes

1
Writing "breed two good parents" without saying "repeat for many generations".
2
Saying selective breeding makes animals adapt — humans select, animals do not choose.
3
Ignoring that inbreeding reduces variation.
4
Confusing selective breeding with genetic engineering — no genes are transferred.

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