Salmon farmers want to produce a strain of salmon that grows more quickly than wild salmon. Explain how selective breeding could be used to produce this faster-growing strain of salmon.

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).

Model answer (4 marks)

1. Identify the fastest‑growing salmon in the existing population.
2. Interbreed those selected individuals.
3. From the offspring, select the fastest‑growing fish.
4. Repeat steps 2–3 over many generations until the strain consistently shows fast growth and breeds true for the trait.

Examiner tips

Use the exact sequence of selection, breeding, and repetition; show the iterative nature.
Mention that the strain must breed true to demonstrate a stable genetic change.
Keep the answer concise – 4 points, one sentence each.
Use UK spelling (e.g., ‘breed’).

Common mistakes

Confusing selection of the fastest fish with breeding the fastest fish with the slowest; students sometimes say ‘breed with wild salmon’. Failing to mention that the process is repeated over many generations. Using vague terms like ‘improve growth’ instead of ‘select fastest‑growing individuals’.

Mark scheme (4 marks)

Select/identify salmon that show the fastest growth (from the existing population)
Breed the selected (fastest-growing) salmon with each other / interbreed the selected individuals
Select the fastest-growing offspring and repeat the process over many generations
Until offspring consistently / reliably show the desirable phenotype (fast growth) / the strain breeds true for fast growth

Key terms in this question

selective breeding