Selective Breeding
Edexcel International GCSE BiologyΒ· 5.10β5.11 (2017 Issue 3 spec)Β· 12 min read
1. The Selective Breeding Processβ β ββββ± 3 min
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Selective Breeding (Artificial Selection)
The process by which humans intentionally select organisms with desirable characteristics to breed together, over many generations, to produce offspring that reliably express those desired traits.
Select parent organisms that show the desired characteristic
Breed the selected parents together
Select the offspring that show the strongest expression of the desired characteristic
Breed these selected offspring together
Repeat the selection and breeding cycle over many generations until all offspring reliably show the desired trait
A farmer wants to produce wheat plants with a higher grain yield. Outline the process he would use using selective breeding.
- 1
- First, the farmer selects existing wheat plants that produce the highest grain yield from his crop.
- 2
- He breeds these high-yield parent wheat plants together, allowing them to pollinate and produce seeds.
- 3
- He grows the seeds into new wheat plants, then selects the offspring that have the highest grain yield.
- 4
- He breeds these high-yield offspring together, and repeats the selection and breeding cycle for many generations.
- 5
- After many generations, all the wheat plants produced will reliably have a much higher grain yield than the original crop.
Exam tip:
Always mention the repeated cycle over many generations in your answers: this is a required mark point for almost all selective breeding process questions.
2. Selective Breeding in Plantsβ β ββββ± 3 min
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Selective breeding is widely used in agriculture to improve crop plants for food production, with traits selected that increase yield, reduce losses to disease, or improve consumer appeal.
Higher yield grain crops (wheat, rice, maize)
Disease-resistant fruit and vegetable crops
Drought-resistant crops for dry growing regions
Ornamental flowers with larger, more colourful blooms for horticulture
A horticulturist wants to develop rose plants that produce deep red, strongly scented blooms. Describe how selective breeding can achieve this.
- 1
- Select parent rose plants that have the deepest red blooms and the strongest scent from an existing population.
- 2
- Cross-pollinate these selected parent roses to produce seeds.
- 3
- Grow the seeds into new rose plants, and select the offspring that have both deep red blooms and strong scent.
- 4
- Breed these selected offspring together, repeating the cycle for many generations until all offspring reliably produce deep red, scented blooms.
Exam tip:
Stick to widely accepted agricultural/horticultural examples for plant selective breeding questions, as these are explicitly listed in mark schemes.
3. Selective Breeding in Animalsβ β ββββ± 3 min
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Selective breeding is used for livestock, companion animals, and working animals to select for traits that are useful for humans or improve animal welfare.
Dairy cattle with high milk yield
Beef cattle with high meat yield
Sheep that produce high quality/quantity of wool
Hens that lay large numbers of eggs
Dogs bred for calm temperament (pets) or working ability (sheepdogs, guide dogs)
A dairy farmer wants to increase the volume of milk his cows produce. Explain how he would use selective breeding to achieve this.
- 1
- Identify the cows in his herd that produce the highest volume of milk, and select a high-yield dairy bull to breed with these cows.
- 2
- Breed the selected high-yield cows and bull together, producing calf offspring.
- 3
- When the female calves reach maturity, select those that produce the highest milk yield to breed with another high-yield bull.
- 4
- Repeat this selection and breeding cycle over many generations, until all cows in the herd reliably produce very high volumes of milk.
4. Disadvantages and Key Comparisonsβ β β βββ± 3 min
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While selective breeding provides many benefits for food production and other uses, it also has important drawbacks that you need to know for the exam, as well as key differences from genetic engineering (a separate topic that is often compared in exam questions).
Gene pool
The total number of different alleles present in a population of a single species.
A key disadvantage of selective breeding is that it reduces the size of the gene pool in the selectively bred population, because only a small number of parent organisms with the desired trait are bred each generation, reducing genetic variation. This leads to an increased risk of the population being wiped out by new diseases, and a higher rate of inherited defects due to inbreeding of closely related individuals.
Feature | Selective Breeding | Genetic Engineering |
|---|---|---|
Mechanism | Uses natural sexual reproduction, selects for existing variation in the species | Directly transfers specific genes from one organism to another, can cross species barriers |
Speed | Slow, takes many generations to produce desired traits | Fast, traits can be added in a single generation |
Inheritance risk | Higher risk of inherited defects due to inbreeding | Lower risk from inbreeding, though other safety risks apply |
Exam tip:
The comparison between selective breeding and genetic engineering is a common 3-4 mark question: learn the table below to gain full marks for these questions.
5. Common Pitfalls
Wrong move:
Only describing 1 or 2 generations of selection
Why:
Mark schemes explicitly require stating the process is repeated over many generations to get full marks.
Correct move:
Always mention that the selection and breeding cycle is repeated over many generations until the trait is reliably expressed in all offspring.
Wrong move:
Confusing selective breeding with genetic engineering
Why:
The two processes are often compared in exam questions, and mixing them up leads to lost marks.
Correct move:
Remember selective breeding uses natural reproduction and existing variation, while genetic engineering directly transfers genes between organisms.
Wrong move:
Forgetting to mention selection of both parents and offspring
Why:
Both steps are required in the process, missing one loses a mark.
Correct move:
Include selecting parents with the desired trait, then selecting the best offspring to breed in every cycle.
Wrong move:
Stating that selective breeding creates new traits
Why:
Selective breeding only selects for existing variation in a population, it does not create new alleles.
Correct move:
State that selective breeding amplifies existing desired traits that are already present in the species.
Wrong move:
Not linking reduced genetic variation to increased disease risk
Why:
This is a standard disadvantage asked for in many questions, and requires a causal link to get full marks.
Correct move:
Explain that a smaller gene pool means fewer individuals will have alleles that confer resistance to new diseases, so the population is at higher risk of being wiped out.
6. Quick Reference Cheatsheet
Category | Key Exam Points |
|---|---|
Process Steps |
|
Plant Examples | High yield crops, disease-resistant crops, ornamental flowers |
Animal Examples | High milk yield cattle, high wool yield sheep, high egg-laying hens, calm temperament dogs |
Disadvantages | Reduced gene pool, lower genetic variation, higher risk of disease/inherited defects |
Vs Genetic Engineering | Slow, natural reproduction, no cross-species gene transfer |
7. Frequently Asked
How many generations of selection are required for full marks?
Mark schemes explicitly require you to state that the selection and breeding cycle is repeated over many generations (not 1 or 2) until the desired trait is reliably expressed in all offspring. Missing this point will lose you a mark.
What is the difference between selective breeding and natural selection?
Selective breeding is controlled by humans to select for traits that are useful to people, while natural selection is driven by environmental pressures that select for traits that improve an organism's chance of survival and reproduction. Selective breeding is also much faster than natural selection for producing populations with uniform traits.
Going deeper
What's Next
Now that you have mastered selective breeding for Edexcel IGCSE Biology, you can move on to the related topics in the biological resources unit. The next core topic is genetic engineering, which is often compared to selective breeding in exam questions, so understanding the differences between the two processes will help you score high marks on comparison questions. You can also review variation and natural selection to consolidate your understanding of how traits are passed between generations, which underpins all selective breeding processes. Make sure to practice past paper questions on selective breeding to apply your knowledge to exam-style scenarios.