A salmon is a vertebrate animal that reproduces sexually. During sexual reproduction, gametes are produced by meiosis and then fuse during fertilisation. Explain why the offspring produced by sexual reproduction in salmon are not genetically identical to one another.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Salmon reproduce sexually. Each female salmon can release thousands of eggs into the water, where they are fertilised by sperm from a male salmon. The fertilised eggs develop into new salmon.
Model answer (5 marks)
1. Gametes are haploid, containing only one set of chromosomes.
2. Meiosis creates genetic variation, so gametes are not identical.
3. Fertilisation is random; any egg can fuse with any sperm.
4. The zygote receives chromosomes from both parents.
5. Different combinations of chromosomes and alleles are passed to each offspring, producing genetic variation.
2. Meiosis creates genetic variation, so gametes are not identical.
3. Fertilisation is random; any egg can fuse with any sperm.
4. The zygote receives chromosomes from both parents.
5. Different combinations of chromosomes and alleles are passed to each offspring, producing genetic variation.
Examiner tips
- Use the exact terms: haploid, meiosis, fertilisation, zygote, alleles.
- Show the chain: meiosis → haploid gametes → random fertilisation → zygote with mixed chromosomes.
- Keep each point brief but complete to hit all five marks.
Common mistakes
- Confusing meiosis with mitosis; not stating haploidy.
- Forgetting that fertilisation is random or that the zygote gets chromosomes from both parents.
- Using vague words like ‘different’ without explaining the mechanism of variation.
Mark scheme (5 marks)
- Gametes are haploid / contain only one copy of each chromosome
- Meiosis produces genetic variation / gametes are not genetically identical to each other
- Fertilisation is random / any egg can be fertilised by any sperm
- The zygote / fertilised egg receives chromosomes from both parents
- Different combinations of chromosomes / alleles are passed to each offspring, producing genetic variation
Key terms in this question
gametes · meiosis · fertilisation
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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