Exam Focus

Exam Focus

Sex determination is tested regularly in AQA Paper 2 and Edexcel 1BI0/1 (Paper 1). Common question formats:

• Genetic diagram: Draw the Punnett square for sex determination, identify gametes, state the probability as a ratio and percentage.
• Explain questions: "Explain why the father determines the sex of the child." Answer must include: mother always gives X, father gives X or Y, Y = male, X = female.
• Higher tier — X-linked disorders: Given a pedigree diagram, identify carriers and affected individuals. Calculate probability of an X-linked condition in sons.
• Common error question: "A couple say the mother determines the baby's sex. Evaluate this claim." Requires a clear correction with genetic explanation.

Edexcel 1BI0 style: Edexcel Paper 1 (1BI0/1) may present sex determination questions with a stimulus — for example, a pedigree chart showing an X-linked condition across three generations, or statistical data on sex ratios in a population. The command word "Suggest" is used when the condition or family pattern is unfamiliar. For X-linked questions, Edexcel mark schemes accept both superscript notation (X^H, X^h) and alternative notations — but using the superscript format is recommended as it makes chromosomal linkage explicit.

Mark-scheme tip: When completing a sex determination genetic diagram, always write the gametes explicitly (X and Y for father; both X for mother) before filling in the Punnett square — these carry method marks.