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5.2Use of Biological Resources

Selective Breeding — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions

10 min read 8 parts Lesson 2 of 7 1 quiz

📚 Edexcel International GCSE Biology 4BI1 Hub / Topical Revision
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Lesson summary

Selective Breeding Edexcel IGCSE Biology 4BI1 notes: 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 brea…

  • Definition
  • Plant method
  • Animal method
  • Natural vs artificial selection
  • Reducing inbreeding
  • Many varieties from one ancestor

Part 1 of 8Key ideas

1
Selective breeding works because the chosen characteristics are controlled by genes that are inherited.
2
It takes many generations, so it is slow, especially for animals with long life cycles.
3
Modern bread wheat was bred for high yield and short stems.
4
Holstein-Friesian cows have been bred for very high milk yield.
5
Reduced variation means fewer alleles in the gene pool.
6
Inbreeding increases the chance of two harmful recessive alleles being inherited together.

Part 2 of 8Learning objectives

1
Define selective breeding.
2
Describe how plants are selectively bred for yield and disease resistance.
3
Describe how animals are selectively bred for food production.
4
Evaluate the advantages and disadvantages of selective breeding.

Part 3 of 8Essential knowledge

Essential knowledge
Definition
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
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
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
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
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
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
In natural selection the environment determines which individuals survive and reproduce, giving features useful for survival; it is usually slow. In artificial selection humans choose the parents for features useful to people; it is faster because only chosen individuals breed.
Reducing inbreeding
Breeders choose several unrelated parents that all show the desired feature, so that closely related animals, such as siblings, do not have to be bred together in later generations.
Many varieties from one ancestor
Selective breeding of one wild cabbage species produced cabbage, cauliflower, broccoli, kale and Brussels sprouts. All dog breeds were bred from wolves by selecting different features.

Part 4 of 8Process & 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 cyclewrite these steps in this order; each step is a marking point.

    1. Choose desired trait
    2. Select best parents
    3. Breed them
    4. Select best offspring
    5. Repeat for many generations

Part 5 of 8Worked examples

1Describe 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.
2All plants are susceptible

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.

Part 6 of 8Summary

    • Selective breeding = humans choosing parents with useful traits.
    • It is repeated over many generations.
    • Plants are bred for yield and disease resistance.
    • Animals are bred for meat, milk and temperament.
    • The main drawback is reduced genetic variation.

Part 7 of 8Keep going

  1. Selective Breeding Edexcel IGCSE Biology 4BI1: what to do next

    These Selective Breeding Edexcel IGCSE Biology 4BI1 notes covered the key ideas, worked examples and exam technique for this topic. Check your Selective Breeding Edexcel IGCSE Biology 4BI1 understanding with the practice questions below, then reread any Selective Breeding Edexcel IGCSE Biology 4BI1 point you were unsure about before moving on.

    Selective Breeding Edexcel IGCSE Biology 4BI1: revision checklist

    • Revise the main Selective Breeding Edexcel IGCSE Biology 4BI1 definitions until you can write them from memory.

Part 8 of 8Compare: Compare

Examples of selective breeding
OrganismDesired characteristicBenefit
WheatHigh yield, short stemsMore grain; less damage from wind
Crop plantsDisease resistanceFewer losses, less pesticide
Dairy cattleHigh milk yieldMore milk per cow
Beef cattleFast growth, muscle massMore meat
SheepFine, thick woolBetter textile quality
Advantages and disadvantages
AdvantagesDisadvantages
Increases yield of foodReduces genetic variation
Can produce disease-resistant cropsPopulation vulnerable to new diseases
Animals can be made more docileInherited defects more likely (inbreeding)
Uses natural reproductionTakes many generations

Key terms

Command terms

Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Selective breeding · Artificial selection · Characteristic · Generation · Yield · Inbreeding · Gene pool · Disease resistance
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm3 → dm3) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.

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