Topic 17.4 · Core
Pedigree Diagrams
A pedigree runs the logic of a genetic diagram backwards: instead of predicting offspring from known parental genotypes, you work out genotypes from a real family’s observed phenotypes. The method is reasoning from evidence, not applying a memorised shortcut — and the shortcuts are exactly where marks get lost.
Reading a pedigree
A pedigree diagram is a family tree used to trace a characteristic across generations. Read the symbol key first, every time — the precise meaning of shaded and unshaded symbols depends on the specific characteristic being represented, and assuming a convention without checking is a common source of errors. A common convention uses a square for a male and a circle for a female, with a shaded symbol showing an individual who displays the characteristic and lines between individuals showing family relationships across generations.
How to work out genotypes from the evidence
Don’t start by guessing every genotype in the diagram. Start with whichever individuals the evidence pins down most securely, then let that information propagate outward:
- Identify which individuals are affected and which are unaffected.
- Identify the parent–offspring relationships in the diagram.
- Apply the stated inheritance model — usually, which allele is dominant and which is recessive.
- Use the individuals whose genotype is forced by their phenotype first.
- Propagate those constraints through the rest of the family.
- Where more than one genotype remains possible for an individual, say so rather than picking one arbitrarily.
For a recessive characteristic, an individual showing the recessive phenotype must be homozygous recessive — there is no other genotype that produces that phenotype. That single fact is usually where a pedigree question actually opens up. For example, if two parents both show the dominant, unaffected phenotype but have a child who is affected, that child must be homozygous recessive (aa). Since each parent supplied one of those two recessive alleles, both parents must themselves carry a recessive allele alongside a dominant one — in other words, both parents must be heterozygous, Aa × Aa — even though neither parent shows the recessive phenotype themselves.
Where the reasoning has to stop
Being affected does not automatically mean an individual is homozygous. For a recessive condition, an affected individual must be homozygous recessive, as above — but for a dominant condition, an affected individual could be homozygous dominant or heterozygous, and a pedigree only lets you choose between the two if other family evidence rules one option out. Don’t claim certainty a diagram doesn’t actually support.
It’s also worth actively avoiding common shortcuts that feel true but aren’t reliable rules — “a recessive characteristic always skips a generation” is not something a pedigree guarantees, and treating it as a rule rather than reasoning from the specific family shown is a fast way to answer confidently and incorrectly. A pedigree has to be reasoned from the actual evidence in that diagram, not from a memorised pattern.
Why this matters beyond the exam
Pedigrees are how the genetics in this chapter actually gets applied to real families: they let you trace how a characteristic, including an inherited condition, has passed through generations, and predict the chance of it appearing again — using only the evidence given, the same genotype-to-phenotype rules covered on the monohybrid inheritance page, and the same forward-reasoning method used to build a Punnett square, just run in reverse.