Genetics & Inheritance
Reproduction and Inheritance · 6 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 17% of your exam marks.
Genetic crosses, Punnett squares, and dominant/recessive allele questions appear on almost every paper.
A family pedigree is a diagram that traces how a particular characteristic (often a genetic disorder) passes through several generations of a family. Pedigree diagrams are useful for working out:
- Whether the characteristic is or
- The probability that a particular family member is affected, or could pass the condition on to their children
Pedigree symbols
Pedigree diagrams use standard symbols:
- Squares represent males; circles represent females
- Filled-in (shaded) shapes represent affected individuals (those showing the characteristic)
- Empty shapes represent unaffected individuals
- A horizontal line between a male and a female means they are a couple who have produced children
- A vertical line going down from a couple connects them to their children, who are shown side by side
Reading a pedigree
To work out whether a trait is dominant or recessive:
- If two unaffected parents have affected children, the trait must be recessive (because both parents are carriers but don't show the trait).
- If every affected child has at least one affected parent, the trait is likely to be dominant.
Using a pedigree to determine genotypes and inheritance pattern
What comes up: given a pedigree diagram, the exam asks you to (1) state the genotypes of named individuals, and (2) draw a genetic diagram for a particular couple to predict offspring outcomes.
Write (two marks for the genetic diagram): 1. State the parental genotypes correctly (e.g. Ff and Ff for two carriers of a recessive condition). 2. Show the gametes each parent can produce (e.g. F or f for each parent), then list or grid the offspring genotypes (FF, Ff, ff) and their phenotypes with the correct ratio.
Watch out: the mark scheme requires both genotypes and phenotypes for the offspring — giving genotypes alone loses the final mark. When determining whether a trait is dominant or recessive from the pedigree, state the key evidence: if two unaffected parents produce an affected child, the trait must be recessive (both parents are carriers, genotype Ff, and the affected child is ff).