Exam Tips: Inherited Disorders

CF is recessive, polydactyly is dominant: Know this cold. Examiners test it directly: "State whether cystic fibrosis is caused by a dominant or recessive allele." One word answer required — recessive.

Carrier definition in three parts: (1) heterozygous, (2) does not show the condition, (3) can pass the recessive allele to offspring. Include all three parts for a full marks definition.

Ethical evaluation needs balance: When asked to evaluate genetic screening, give at least one benefit AND one concern. Examiners want to see that you can weigh up both sides. Common benefits: informed decisions, prepare for care. Common concerns: discrimination, autonomy, embryo selection ethics.

State the probability correctly: After a Punnett square, state the answer as fraction (1/4), ratio (1:3), and percentage (25%) if asked. Read the question carefully — "probability" and "ratio" are different things.

CF effects — both lungs AND digestion: Many students only state the lung effects (thick mucus, breathing difficulty). For a complete answer, also mention that mucus blocks pancreatic ducts, preventing digestive enzymes from reaching the small intestine.

Edexcel "Suggest" questions on novel disorders: Edexcel 1BI0/1 may present an unfamiliar inherited condition in a stimulus passage and ask you to "Suggest" whether it is dominant or recessive. Look for clues in the family data: if two unaffected parents have an affected child → recessive. If every affected individual has an affected parent → likely dominant. Apply the same logic you use for CF and polydactyly.