Exam Focus

Exam Focus

Genetic crosses and Punnett squares are tested in almost every AQA Paper 2 and Edexcel 1BI0/1 (Paper 1). This is one of the highest-mark topics in Unit 7. Common question formats:

• 2-mark Punnett square: Complete a cross, state the ratio, and give the probability. Always show gametes clearly — they are method marks.
• 3-mark explanation: "Explain how two parents with a dominant phenotype can have a child with the recessive phenotype." Requires: heterozygous parents, carrier definition, recessive needs two copies.
• 1-mark probability: Convert Punnett square result to fraction or percentage. Know 1/4 = 25%, 1/2 = 50%, 3/4 = 75%.
• 6-mark extended response: Occasionally tested — describe the mechanism of dominant/recessive inheritance with reference to genotype and phenotype throughout.

Edexcel 1BI0 style: Edexcel Paper 1 (1BI0/1) frequently presents genetic inheritance questions with a stimulus — for example, a family pedigree chart, a description of a breeding programme, or data on phenotype frequencies in a population. You are expected to apply Punnett square knowledge to the specific scenario given. The command word "Suggest" is used when the context is novel — use your genetics knowledge to reason through the unfamiliar case. Edexcel mark schemes use "Allow..." to credit alternative correct allele notation, but your Punnett square method must still show gametes clearly.

Mark-scheme language: When completing a Punnett square, always state the genotype ratios AND the phenotype ratio (e.g., "3 brown : 1 blue", not just "3:1"). Examiners want to see you connect genotype to phenotype.