6.2.3Inheritance, Variation and Evolution
Selective Breeding
10 min read 8 parts Lesson 10 of 16 1 quiz
Section 1 of 8Selective Breeding
1Selective breeding relies on genetic variation
Selective breeding relies on genetic variation there must be individuals with the useful alleles to choose from.
2Plants
Plants breeders choose parents with desired traits (e.g. high yield, disease resistance), transfer pollen by hand between them, and cover flowers with bags to stop other pollen reaching them.
3
Seeds from the cross are grown, the best offspring are selected and crossed again, for many generations.
4Examples in plants
Examples in plants wheat with shorter, stronger stems that do not fall over and larger grain yield; crops resistant to disease; drought-tolerant varieties.
5Animals
Animals breeders choose a male and female with desired traits (e.g. high milk yield, fast growth), mate them, and select the best offspring to breed again; artificial insemination spreads semen from one good bull to many cows.
6Examples in animals
Examples in animals dairy cattle producing more milk; beef cattle with more muscle; sheep with better wool; hens laying more eggs.
7
Because only a few chosen individuals breed, many alleles are lost from the gene pool, reducing variation.
8
Low variation means the population may not contain resistant individuals if a new disease or climate change occurs, so the whole crop/herd could be lost.
9
Inbreeding makes it more likely that two copies of a harmful recessive allele are inherited, causing genetic disorders or reduced fertility.
10
Selecting for one trait can cause welfare problems, e.g. very fast-growing broiler chickens with leg problems, or cows with udder strain.
11
Selective breeding takes many generations, so it is slow compared with genetic modification, and can only use alleles already present in the species.
12
It differs from natural selection because humans choose, not the environment, and chosen traits may not help survival in the wild.
13Classic examples
A single wild brassica species has been bred for different parts into cabbage, cauliflower, broccoli, kale and Brussels sprouts; all domestic dog breeds descend from wolves through selection for temperament, size and appearance.
14Limiting inbreeding
Breeders often start with several unrelated parents that all show the desired trait, so siblings need not be mated in the next generation; this keeps more alleles in the gene pool and lowers the risk of inherited defects.
Section 2 of 8Process & sequence
- Decide which characteristic is wanted (e.g. high milk yield).
- Choose parents that show the characteristic most strongly from a varied population.
- Breed them together (for plantscross-pollinate by hand, covering flowers to prevent unwanted pollination).
- From the offspring, select those showing the characteristic most strongly.
- Breed these selected offspring together.
- Repeat over many generations until all offspring reliably show the characteristic.
Breeding disease-resistant wheatwrite these steps in this order; each step is a marking point.
- Find a wheat variety with high yield and a variety with disease resistance.
- Remove anthers from one plant and transfer pollen from the other by brush; bag the flowers.
- Grow the seeds; select offspring with both high yield and resistance.
- Cross the selected offspring with each other.
- Repeat for many generations until the new variety breeds true.
Section 3 of 8Compare
| Feature | Selective breeding | Natural selection |
|---|---|---|
| Who selects | Humans | The environment (selection pressure) |
| Selected for | Traits useful to humans | Traits improving survival and reproduction |
| Speed | Relatively fast | Usually slow |
| Effect on variation | Reduces genetic variation | Maintains more variation |
| Organism | Desired characteristic |
|---|---|
| Wheat / rice | High yield, short stems, disease resistance, drought tolerance |
| Dairy cattle | High milk yield, docile temperament |
| Beef cattle | Fast growth, high meat yield |
| Sheep | Fine, dense wool |
| Chickens | Many eggs (layers) or fast growth (broilers) |
| Crop plants | Resistance to pests / disease; larger fruit |
Section 4 of 8Worked examples
90%
Mean milk yield in a herd rose from 4200 dm3 per cow per year to 7980 dm3 after 9 generations of selective breeding. Calculate the percentage increase and the mean increase per generation.
- Increase = 7980 − 4200 = 3780 dm3.
- Percentage = 3780 ÷ 4200 × 100 = 90%.
- Per generation = 3780 ÷ 9 = 420 dm3 per generation.
25%
Two dogs are both carriers (Hh) of a recessive allele h for hip disease. What percentage of puppies would be expected to have the disease? Explain why this is more likely with inbreeding.
- Hh × Hh → HH : Hh : hh = 1 : 2 : 1, so hh = 1/4 = 25%.
- Close relatives are more likely to carry the same recessive allele, so inbreeding increases the chance of two carriers mating.
Section 5 of 8Keep going
Selective Breeding Edexcel O Level Biology: what to do next
These Selective Breeding Edexcel O Level Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Selective Breeding Edexcel O Level Biology understanding with the practice questions below, then reread any Selective Breeding Edexcel O Level Biology point you were unsure about before moving on.
Selective Breeding Edexcel O Level Biology: revision checklist
- Revise the main Selective Breeding Edexcel O Level Biology definitions until you can write them from memory.
- Practise Selective Breeding Edexcel O Level Biology exam questions under timed conditions.
- Link each Selective Breeding Edexcel O Level Biology idea to a diagram or a worked example.
- List the common Selective Breeding Edexcel O Level Biology mistakes and how to avoid them.
- Practice: Selective Breeding Edexcel O Level Biology practice questions
- Previous lesson: 5.1 Food Production — O Level Notes
- Next lesson: 5.3 Genetic Modification (Genetic Engineering) — O Level Notes
- Course: Edexcel O Level Biology — all units
- Official syllabus: Pearson Edexcel International GCSEs
Section 6 of 8Key terms
Section 7 of 8Command 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.
Section 8 of 8Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Selective breeding (artificial selection) · Characteristic (trait) · Phenotype · Genotype · Allele · Variation · Inbreeding · Gene pool · Hybrid · Natural selection · Cross-pollination · Yield
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)
1
Use specification keywords exactly (e.g. enzyme–substrate complex, active site, complementary).
2Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
3Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
4
Convert units carefully (mg → g, cm3 → dm3) and show working for calculations.
5Avoid vague words
“it”, “stuff”, “goes up”, “energy is made”. Name the substance and the process.
6
Extended (6-mark) responses are marked on content AND a logical, linked line of reasoning.
Words to avoid
1
the farmer changes the genes of the animalthe farmer selects parents with desired characteristics and breeds them, so the desired alleles become more common
2
breed the best animals oncerepeat the selection and breeding over many generations
3
selective breeding creates new genesselective breeding only uses alleles already present in the population
4
inbreeding causes mutationsinbreeding increases the chance of inheriting two copies of a harmful recessive allele
5
selective breeding makes the animals strongerit reduces genetic variation, so the population is more vulnerable to new diseases