Edexcel IGCSE Biology · Spec 3.24
Family Pedigrees
How to read and interpret family pedigree diagrams to follow inherited characteristics.
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Family Pedigrees
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Explained
Reading a family pedigree
A pedigree is a family tree that follows one characteristic through several generations. The symbols are fixed and simple: squares are males, circles are females, and a shaded symbol means that person is affected. Horizontal lines join partners, vertical lines lead down to their children, and each row is a generation.
Working out whether a condition is recessive
The most useful clue in any pedigree is a child who is affected when neither parent is. That cannot happen with a dominant condition, because a dominant allele always shows. It can only happen if both parents carried a hidden recessive allele and each passed it on.
So if you find unaffected parents with an affected child, the condition is recessive and both parents must be carriers. That single observation answers a large share of pedigree questions on its own.
Carriers
A carrier has one recessive allele and one dominant allele, written Aa, and does not show the condition. An affected person with a recessive condition must be aa, because one copy of the dominant allele would mask it.
You can therefore fill in some genotypes with certainty and others only partly. An unaffected person in a family with a recessive condition is either AA or Aa, and unless the diagram tells you more, writing A followed by a dash is the honest answer. Guessing between them loses marks that admitting the uncertainty would keep.
Working through a pedigree in order
- Decide whether the condition is dominant or recessive, using unaffected parents with an affected child.
- Write in the genotypes you know for certain, starting with every affected person.
- Work back to the parents of anyone affected. Each of them must have supplied one recessive allele.
- Only then work forwards to the probabilities the question asks for.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Biology show that full marks on the associated genetic diagram questions require four things: the parent genotypes, the gametes those parents can produce, the offspring genotypes, and the ratio of phenotypes.
Responses recorded as scoring three rather than four typically had the genotypes right but left the gametes out. If you are asked to draw a genetic diagram, include the gametes row even when it feels obvious, because it carries its own mark.
Giving a probability
Probabilities from a pedigree come from a Punnett square, not from counting the children in the diagram. Two carrier parents have a one in four chance of an affected child at every pregnancy, regardless of how many affected children they already have.
Questions sometimes include a family where three of four children are affected, precisely to see whether you will answer from the diagram instead of from the genetics. The chance stays one in four.
Spec 3.24
What you need to know
- Read the symbols in a pedigree chart
- Follow a characteristic through a family
- Work out whether a condition is recessive
Active recall
Quick check
Answer each question before opening the answer.
How can you tell an allele is recessive from a pedigree?
Two unaffected parents have an affected child, showing they were carriers of a hidden recessive allele.
What does a pedigree diagram show?
How a characteristic is inherited through the generations of a family.
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