Exam Tips: Genetic Inheritance

Always show your gametes: Before filling in the Punnett square, write the gametes along the top and side. These carry method marks — even if you fill the grid incorrectly, you can still earn marks for correct gametes.

State both ratio and probability: After completing a Punnett square, give the genotype ratio (e.g., 1 BB : 2 Bb : 1 bb) AND the phenotype ratio (e.g., 3 brown : 1 blue). Then give the probability as a fraction and percentage.

Dominant does not mean common: This is one of the most common misconceptions — explicitly say "dominant means it is expressed when one copy is present" rather than "dominant means it is more frequent".

Carrier definition: A carrier is always heterozygous (one dominant, one recessive allele) for a recessive condition. They do not show the condition. If asked to define "carrier", include all three elements: heterozygous, does not show symptoms, can pass the recessive allele to offspring.

Letter choice matters: Use letters that look different in upper and lower case. Avoid letters like C/c, O/o, P/p where upper and lower can look similar. Good choices: B/b, T/t, F/f, H/h.

Edexcel pedigree chart questions: Edexcel 1BI0/1 regularly uses pedigree (family tree) diagrams as stimulus material. From the diagram, you may be asked to deduce whether a condition is dominant or recessive, identify carrier individuals, or calculate probability for a future child. Work through the logic step by step: if two unaffected parents have an affected child, the condition must be recessive.