By the end of this lesson
- Describe the stages of selective breeding in plants.
- Give examples of desired characteristics in crop plants.
- Evaluate the benefits and risks of selective breeding.
BIG QUESTION
How did a wild grass become modern wheat?
Farmers repeatedly chose and bred from plants with larger grains and higher yields over thousands of years.
The process
In selective breedingHumans choosing parents with desired characteristics to breed, then selecting the best offspring, over many generations. (artificial selection), humans, not the environment, decide which individuals reproduce. Breeders choose parent plants that show a desired characteristic, cross-pollinate them and grow the seeds.
The offspring that show the characteristic most strongly are selected as the next parents. Repeating this over many generations increases the frequency of the alleles for the desired characteristic.
Desired characteristics
Examples include high yieldThe amount of useful product, such as crop mass, obtained., disease resistance, drought or frost tolerance, short stems that resist wind damage, better flavour and earlier ripening.
Characteristics can be combined: a high-yield variety may be crossed with a disease-resistant variety and offspring with both features selected.
Limitations and risks
Selective breeding is slow because each generation must grow and flower. It can only work with alleles already present in the species or a closely related one.
Repeated breeding from similar plants reduces genetic variationDifferences between individuals of the same species caused by differences in their alleles.. A uniform crop may be vulnerable if a new disease or climate change affects all plants in the same way.
Selective breeding in crops
Each characteristic benefits the farmer or consumer in a different way.
| Desired characteristic | Example | Benefit |
|---|---|---|
| High yield | Larger wheat grains. | More food per field. |
| Disease resistance | Rust-resistant wheat. | Less crop loss and less fungicide use. |
| Short stems | Dwarf wheat varieties. | Less wind damage; more energy into grain. |
| Tolerance | Drought-tolerant varieties. | Crops survive in drier conditions. |
Language that earns precision
- selective breeding
- Humans choosing individuals with desired characteristics to breed together.
- desired characteristic
- A trait chosen by the breeder because it is useful.
- variety
- A distinct type of crop plant produced by breeding.
- genetic variation
- Differences in alleles between individuals of a species.
Common misconceptions
Claim: Selective breeding creates new genes.
Correction: It selects from alleles already present; new alleles arise only by mutation.
Claim: One generation of breeding gives the final variety.
Correction: Selection is repeated over many generations.
Claim: Selective breeding is the same as genetic modification.
Correction: Selective breeding crosses whole organisms of the same or related species; it does not insert genes directly.
Match the answer to the command word
Use the sequence: choose parents with the characteristic, cross them, select the best offspring, repeat over many generations.
Balance improved yield or resistance against reduced genetic variation and the time taken.
Recall, apply, then justify
Describe how a farmer could use selective breeding to produce wheat plants that are resistant to a fungal disease.
Hint
Parents, cross, select, repeat.
Suggested marking points
- Choose parent plants that show resistance to the disease.
- Cross-pollinate / breed these plants together.
- Select offspring that show the greatest resistance.
- Repeat over many generations.
Suggest one disadvantage of growing a single selectively bred variety across a whole region.
Hint
Think about genetic variation.
Suggested marking points
- Low genetic variation in the crop.
- A new disease or environmental change could affect / destroy all plants.
A breeder has a high-yield wheat variety that is not drought tolerant and a low-yield wild wheat that is. Explain how a variety with both features could be produced and why it might take many years.
Planning hint
Cross the two, then select repeatedly.
What a strong design includes
- Cross the high-yield variety with the drought-tolerant wild wheat.
- Grow the offspring and test them under dry conditions and for yield.
- Select offspring showing both characteristics as the next parents.
- Repeat selection over many generations until the features are reliable.
- Each generation takes a growing season, so the process is slow.
- Offspring inherit a mix of alleles, so many may lack one of the features.