Show model answer and explanation
2
The symbols A and a represent two alleles.
Class 9 · English Medium · Cambridge · 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
Show your working on paper and enter your answer here. If it does not match, compare the model and try again.
Show model answer and explanation
1/4
One of four equally weighted combinations is aa.
Show model answer and explanation
halve
It forms haploid cells from a diploid starting cell in this context.
Write in your own words. A guardian or teacher should review this response.
Show model answer and explanation
Each conception is a separate chance event under the model; the fraction describes probability, not a fixed sequence.
Explain the repeated-event meaning.
Write in your own words. A guardian or teacher should review this response.
Show model answer and explanation
Heritable resistant variants survive and reproduce more successfully under selection.
Avoid purposeful adaptation claims.
Show model answer and explanation
0.32
2 × 0.8 × 0.2 = 0.32.
Write in your own words. A guardian or teacher should review this response.
Show model answer and explanation
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.
Review with a guardian or teacher
- State dominance assumptions.
- Show gametes.
- Explain probability and the model’s limits.
Keep your work on paper or in a local file. This page does not upload it or automatically grade the project.
Teacher-led lessons, schedules and prices for this outline are upcoming. You can use any starter practice above now without booking.
Your next step
Solo practice works without an account. Sign in to save a practice record to your learner profile.
Keep practising
Exact lesson links and related activities are labelled separately. Return-to-course links retain your selected learning path.