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2
The symbols A and a represent two alleles.
Class 11 · English Medium · Pearson Edexcel · Biology
Reproduction, inheritance and variation
Use meiosis, inheritance and selection to explain variation.
Shared cross-level core and stretch unit. Start with the core questions, then use the stretch example for deeper work. The same unit is available in related class and board pathways; it is not a full grade-specific or examination-specification course. A teacher should select the appropriate depth and further topics.
Learn the idea
Genes are stretches of DNA; alleles are alternative versions at a locus. Meiosis forms cells with reduced chromosome number and contributes variation. A Punnett square gives model probabilities, not a guaranteed family outcome. Natural selection depends on heritable variation and differences in reproductive success.
Worked example
For Aa × Aa in a simple single-gene dominant/recessive model, genotype possibilities are AA, Aa, Aa, aa. The model probability of aa is 1/4; it does not force one affected offspring in every four births.
- State the inheritance model.
- List possible gametes and combinations.
- Explain probabilities without treating them as a schedule.
Stretch example
For a population meeting Hardy–Weinberg assumptions, p and q are the proportions of two alleles: p+q=1 and genotype frequencies are p², 2pq and q². If q=0.2, the model heterozygote fraction is 0.32, meaning about 32 of 100 individuals in a comparable large population under those assumptions. This is an expected population proportion under assumptions, not an individual diagnosis.
Your written practice
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1/4
One of four equally weighted combinations is aa.
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halve
It forms haploid cells from a diploid starting cell in this context.
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Each conception is a separate chance event under the model; the fraction describes probability, not a fixed sequence.
Explain the repeated-event meaning.
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Heritable resistant variants survive and reproduce more successfully under selection.
Avoid purposeful adaptation claims.
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0.32
2 × 0.8 × 0.2 = 0.32.
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Random mating and absence of selection; the full idealised model also excludes mutation, migration and substantial genetic drift.
State valid assumptions and avoid presenting them as true of every real population.
Inheritance model
Create a fictional single-gene cross and explain its possible outcomes.
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- State dominance assumptions.
- Show gametes.
- Explain probability and the model’s limits.
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