Apply selective breeding examples
Explain disease-resistant crops, inbred and hybrid maize, and selection for dairy-cattle milk yield.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers and calculations are self-checked; this is not fieldwork or practical-skills certification.
Before you begin
- Populations contain inherited variation, and alleles can be transmitted from parents to offspring.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What can crossing distinct maize lines produce?
What you will learn
- Outline how disease resistance is introduced into wheat and rice
- Explain inbreeding and hybridisation for vigorous, uniform maize
- Explain selection for improved milk yield in dairy cattle
Original selection schematic. Population counts and scenarios are supplied teaching models; they are not field observations.
| Stage or record | Meaning |
|---|---|
| Before cross | Selected parents with the stated feature |
| Cross | Reproduction produces the next generation |
| After cross | Offspring vary and need assessment |
| Distinction | Breeding is not automatically gene editing |
Build the evolutionary explanation
Disease-resistant wheat or rice varieties can be produced by selecting plants that carry resistance and breeding them into crop lines. Maize inbreeding produces more homozygous, uniform lines; hybridisation then crosses distinct lines to produce vigorous, uniform hybrids through heterosis. Dairy breeders select cattle with high milk yield and use breeding values and progeny performance to guide subsequent crosses. These are applications of artificial selection.
Interpret data with care
High yield or hybrid vigour can involve trade-offs, and the result depends on genotype, management and environment. Avoid claiming that every hybrid is superior or that one season’s phenotype alone proves inherited value.
Pause and explain
How can dairy breeders use milk-yield records without treating them as a perfect measure of genotype?
Worked example
Why might breeders inbreed maize lines before crossing them?
Show the worked solution
- Inbreeding increases homozygosity and line uniformity.
- Distinct lines can then be crossed to produce hybrids.
Answer Inbreeding helps produce uniform parental lines, which can be crossed to create hybrid maize with vigorous, more uniform offspring.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Outline how a breeder can introduce disease resistance into a wheat variety.
Give me a hint
Identify the population-level variable and sequence the mechanism.
Compare my reasoning
- Identify and select resistant plants.
- Cross and repeatedly select offspring with resistance and desired crop traits.
Select plants with heritable disease resistance, cross them with the target variety and select offspring that retain resistance and useful crop characteristics over generations.
Look for these in your work
- Population process identified
- Cause and consequence linked
Explain the sequence of inbreeding and hybridisation in producing vigorous maize.
Give me a hint
Use the supplied sample or condition to justify the conclusion.
Compare my reasoning
- Inbreed to form uniform lines.
- Cross distinct selected lines to produce hybrids.
Inbreeding produces relatively homozygous, uniform parental lines. Hybridising distinct lines can produce uniform F1 maize with hybrid vigour.
Look for these in your work
- Evidence interpreted
- Assumptions stated
How can dairy breeders use milk-yield records without treating them as a perfect measure of genotype?
Give me a hint
Apply the population model to the new example and qualify its limits.
Compare my reasoning
- Records are affected by age, nutrition and management.
- Compare relatives or use breeding values and progeny data.
Milk yield is influenced by genotype and environment. Breeders can compare standardised records, relatives and progeny performance to estimate inherited breeding value more reliably.
Look for these in your work
- Model applied
- Limits preserved
Common mistakes
- Confusing inbreeding with hybridisation
- Saying a hybrid’s performance is guaranteed in every environment
What can you explain now?
What can crossing distinct maize lines produce?
Compare with the explanation
Vigorous, uniform hybrid varieties
The result is associated with hybridisation of selected lines.
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Make it stick
What are the distinctive breeding strategies for wheat/rice, maize and dairy cattle?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.