Start a short session
Check your answer after each question. Use a calculator where needed.
Genetic diagrams · independent outcomes
Inheritance probabilities and genetic ratios
Use Punnett tables, independent dihybrid probabilities, sex-linked reference groups and expected phenotype counts. Each question states its genetic assumptions.
Method and worked examples ↓Learn the method
How to approach these questions
- Write parental genotypes and the possible gametes before combining them.
- Keep genotype and phenotype distinct. Complete dominance makes AA and Aa share a phenotype, not a genotype.
- For independent loci, multiply the relevant probabilities. Linkage can invalidate a simple product.
- Check whether a sex-linked question refers to all offspring or only sons/daughters.
- A predicted ratio describes probabilities; a finite sample need not match it exactly.
Worked examples
Cross Aa × Aa with complete dominance. Find the probability of aa and of the dominant phenotype.
The four combinations are AA, Aa, Aa and aa. P(aa) = 1/4 = 0.25. P(dominant phenotype) = 3/4 = 0.75.
Cross AaBb × AaBb with independent loci. Find the probability of both recessive phenotypes.
P(aa) = 1/4 and P(bb) = 1/4. Independence gives P(aabb) = 1/4 × 1/4 = 1/16 = 0.0625.
Keep practising
Topic map and study list · Practical data guides · Active recall decks
Course resources
AQA A-level Biology · OCR A A-level Biology · Textbook answers
Skill coverage follows the common A-level science subject content. Check your board’s current requirements: AQA 7402 and OCR A H420.