B35: Selective Breeding
Selective breeding and its consequences
Key Definitions
Selective breeding (artificial selection): The process by which humans choose organisms with desirable characteristics and breed them together over many generations to enhance these traits in the population.
The Process of Selective Breeding
1. Choose parents with the desired characteristics
2. Breed them together
3. Select the best offspring with the desired traits
4. Breed these offspring together
5. Repeat over many generations
2. Breed them together
3. Select the best offspring with the desired traits
4. Breed these offspring together
5. Repeat over many generations
Each generation, the desired characteristic becomes more common and more pronounced in the population.
Examples of Selective Breeding
Example 1: Dairy Cattle
Farmers have selectively bred cows to produce higher milk yields. Over many generations, cows that produced the most milk were chosen as parents. Modern dairy cows produce significantly more milk than wild ancestors.
Desired characteristic: High milk production
Example 2: Disease-Resistant Crops
Wheat and other crops have been selectively bred for disease resistance. Plants that survive disease outbreaks are used as parents for the next generation. This reduces the need for chemical pesticides and increases food security.
Desired characteristic: Resistance to fungal, bacterial, or viral diseases
Example 3: Domestic Dogs
Dogs have been selectively bred from wolves for thousands of years. Different breeds were developed for different purposes:
- Border collies — bred for intelligence and herding ability
- Greyhounds — bred for speed and hunting
- Labradors — bred for gentle temperament and retrieving
- Pugs — bred for appearance (flat face), leading to health problems
Example 4: Food Crops
Many food crops have been selectively bred to improve quality and yield:
- Carrots — originally white/purple, selectively bred to be orange with higher beta-carotene
- Tomatoes — bred for larger fruit size and better flavour
- Wheat — bred for larger grain heads and shorter stems (less likely to blow over)
Problems with Selective Breeding
Selective breeding reduces the genetic variation in a population. This can lead to inbreeding (breeding closely related individuals), which increases the risk of inherited disorders and makes the population more vulnerable to disease.
| Problem | Explanation | Example |
|---|---|---|
| Reduced genetic variation | Breeding only individuals with desired traits reduces the gene pool, making the population less able to adapt to changes | Banana crops are all genetically identical — one disease could wipe them all out |
| Inbreeding depression | Breeding closely related individuals increases the chance of offspring inheriting two copies of harmful recessive alleles | Cheetahs have very low genetic diversity due to past population bottlenecks |
| Inherited disorders | Many pedigree dogs suffer from health problems caused by selective breeding | Hip dysplasia in German Shepherds, breathing problems in Pugs, spinal problems in Dachshunds |
| Vulnerability to disease | Low genetic variation means if one individual is susceptible to a disease, all are likely to be | Irish potato famine — all potatoes were genetically similar, so blight destroyed the entire crop |
Comparison: Selective Breeding vs Natural Selection
| Feature | Selective Breeding | Natural Selection |
|---|---|---|
| Who chooses? | Humans choose which organisms breed | Nature — environment determines which organisms survive and reproduce |
| Purpose | To produce traits useful/desirable to humans | To produce traits that increase survival in the environment |
| Speed | Faster — humans intensify the selection | Slower — depends on environmental pressures |
| Traits selected | May not be beneficial for survival (e.g. pug’s flat face) | Always beneficial for survival in that environment |
| Genetic variation | Decreases (narrow gene pool) | Maintained or increased |
Practice Questions
Q1: Foundation Describe the process of selective breeding in four steps.
Q2: Foundation Give two examples of selective breeding in agriculture and state the desired characteristic in each case.
Q3: Foundation Explain why selective breeding can lead to health problems in pedigree dogs.
Q4: Higher Explain the difference between selective breeding and natural selection.
Q5: Higher A farmer selectively breeds wheat for disease resistance over many generations. Explain why this wheat crop could still be vulnerable to a new disease.
Answers
- 1. Choose parents with the desired characteristics from the population. 2. Breed them together. 3. From the offspring, select those with the best expression of the desired trait. 4. Breed these offspring together and repeat the process over many generations.
- 1. Dairy cattle — selectively bred for high milk yield. 2. Disease-resistant wheat — selectively bred for resistance to fungal diseases (or larger grain heads, shorter stems).
- Selective breeding reduces genetic variation and involves inbreeding (mating closely related dogs). This increases the chance of offspring inheriting two copies of harmful recessive alleles, leading to inherited disorders such as hip dysplasia in German Shepherds and breathing problems in Pugs. Traits selected for appearance (e.g. flat faces) may themselves cause health issues.
- In selective breeding, humans choose which organisms reproduce based on traits desirable to humans. In natural selection, the environment determines which organisms survive and reproduce — those best adapted to the environment pass on their alleles. Selective breeding is faster and may produce traits not beneficial for survival, while natural selection produces traits that improve survival in the environment. Selective breeding reduces genetic variation; natural selection maintains it.
- Selective breeding for disease resistance against specific diseases reduces the gene pool — the wheat plants become genetically similar. While they are resistant to the original disease, the lack of genetic variation means there may be no individuals with resistance to a new, different disease. If the new disease arrives, all the plants could be susceptible, potentially wiping out the entire crop.
Exam Tips
- Don’t confuse selective breeding with genetic engineering. Selective breeding uses existing variation and breeding over generations; genetic engineering directly changes DNA by inserting genes from another species
- When asked about problems of selective breeding, always mention reduced genetic variation AND its consequences (inbreeding, inherited disorders, vulnerability to disease)
- For the process, always mention that it takes many generations — it is not a quick fix
- When comparing with natural selection, make it clear that humans do the selecting in selective breeding, while the environment does the selecting in natural selection
🔢 Maths Skills
Mathematical Skills
Interpreting data on selective breeding outcomes: for example, if milk yield per cow increases from 4,000 L/year in 1950 to 10,000 L/year in 2020, the percentage increase = (10,000 − 4,000) / 4,000 × 100 = 150%.
⚠️ Common Misconceptions
Watch Out!
Students often think selective breeding is the same as natural selection. Wrong: Selective breeding = natural selection Correct: In selective breeding, humans choose which organisms reproduce; in natural selection, the environment determines survival and reproduction
Students often think selective breeding has no downsides. Wrong: Selective breeding only has benefits Correct: It reduces genetic variation, increases inbreeding, and raises the risk of inherited disorders (e.g. hip dysplasia in pedigree dogs)
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the process of selective breeding and discuss its ethical concerns.
Selective breeding involves: choosing parents with desired characteristics from a population; breeding them together; selecting the offspring with the best expression of the desired trait; breeding these offspring together; repeating over many generations. This increases the frequency of the desired alleles in the population. Ethical concerns include: reduced genetic variation makes populations vulnerable to disease (e.g. banana crisis from Panama disease); inbreeding increases the risk of inherited disorders such as hip dysplasia in German Shepherds and breathing problems in Pugs; traits selected for appearance may cause suffering (e.g. flat faces causing breathing difficulties); some argue it is wrong to breed animals for human preferences at the expense of their welfare.
Mark scheme: 1 mark for each process step (up to 3), 1 mark for each ethical concern explained (up to 3)
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Data shows that the genetic diversity of cheetahs is extremely low — individuals share 95% of their DNA, compared to 80% in most mammals. Explain how past selective pressures (natural, not human) caused this. Evaluate whether introducing genetically different cheetahs from different regions would help the species' long-term survival.
📝 Exam Questions by Topic
🎬 Video Resources
Ready to ace your GCSE Biology exams?
Get the best revision books and guides to boost your grades.