Explain why it can take many generations of selective breeding before a plant variety with a consistently high sugar content is produced.

Pearson Edexcel International GCSE Biology (4BI1) — 5.2 Selective breeding · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Sugar beet is a crop plant grown for its high sugar content. Farmers use selective breeding to try to increase the sugar content of sugar beet plants over time.

Model answer (4 marks)

1. Only the plants that have the highest sugar content are chosen as parents for the next generation.
2. These selected plants are cross‑pollinated to produce offspring.
3. The offspring show variation in sugar content; not all inherit the high‑sugar trait.
4. The process of selecting the highest‑sugar plants and breeding them must be repeated over many generations to raise the frequency of the desirable trait in the population.

Examiner tips

Use the exact wording from the mark scheme (e.g., "selected", "cross‑pollinated", "variation", "repeated over many generations").
Show the logical sequence: selection → breeding → variation → repetition.
Keep each point concise and separate with a number or bullet.

Common mistakes

Confusing selection with breeding; writing that all offspring inherit the trait.
Using vague terms like "good" instead of "highest" or "most".
Omitting the need for repeated generations or the role of variation.

Mark scheme (4 marks)

Only plants with the highest sugar content are selected / chosen as parents for breeding
The selected plants are bred together / cross-pollinated to produce offspring
The offspring show variation in sugar content / not all offspring inherit the desirable characteristic
The process of selecting and breeding the highest-sugar offspring must be repeated over many generations to increase the frequency of the desirable characteristic / phenotype in the population

Key terms in this question

selective breeding