Explain why selective breeding of crop plants can reduce genetic diversity and suggest why this reduction in genetic diversity could be a disadvantage for the crop population.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Farmers have selectively bred wheat plants for thousands of years to produce high-yielding varieties. Modern wheat crops are grown across large areas and are often genetically very similar to one another.
Model answer (5 marks)
Selective breeding is the deliberate choice of plants with desirable traits to reproduce, so only a few alleles are passed on. Consequently the gene pool shrinks and, over many generations, the crop becomes more genetically similar and homozygous. If a new disease or pathogen appears, most or all plants may be susceptible because they lack resistance alleles. Likewise, a lack of genetic variation means the population cannot adapt to changing environmental conditions, as natural selection has no variation to act upon.
Examiner tips
- Use the phrase "selective breeding" and explain it involves choosing plants with desired traits
- Show the reduction of alleles and increased homozygosity
- Link reduced diversity to vulnerability to disease and inability to adapt
- Use clear, exam‑style language and structure the answer into two parts
Common mistakes
- Confusing selective breeding with random mutation
- Failing to mention the loss of alleles or increased homozygosity
- Not explaining the link between low diversity and disease susceptibility or environmental change
Mark scheme (5 marks)
- Selective breeding involves humans choosing organisms with desired characteristics to breed together
- Only a small number of alleles / gene variants are passed on to offspring (reducing the gene pool)
- Over successive generations the population becomes more genetically similar / homozygous
- If a new disease or pathogen emerges, most/all plants may be susceptible because they lack the alleles for resistance
- The population cannot adapt to changed environmental conditions because there is insufficient genetic variation for natural selection to act upon
Key terms in this question
selective breeding · genetic diversity
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain / Suggest" question →
- Decode the mark scheme abbreviations →