IGCSE Biology / Genes and inheritance
Reading a pedigree
Edexcel International GCSE Biology (4BI1), chapter 18: Genes and inheritance, specification point 3.24. Free lesson.
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A pedigree is a family tree that uses standard symbols to show who has a condition. Reading its patterns can show whether the condition is recessive and help you work out genotypes and probabilities.
How to read the symbols
Each symbol gives information about one family member. Lines show relationships, so you can follow how a condition may pass from parents to children. Generations are often labelled with Roman numerals and individuals are numbered across each generation. A circle represents a female; a square represents a male. A shaded symbol means the person shows the condition. An unshaded symbol means they do not. A horizontal line joins a couple. A vertical line down from it shows their children, listed from left to right. A label such as II-3 means the third individual in generation two.
How to identify a recessive condition
Use the pattern of affected and unaffected family members, not simply how many people have the condition. An unaffected person does not show it. If two unaffected parents have an affected child, the condition is recessive: the parents must each have passed on a recessive allele. First ask: do two unaffected parents have an affected child? If yes, the condition is recessive. Second ask: is the condition on the X chromosome? Look for whether it affects males much more often.
How to deduce the parents' genotypes
For a recessive condition, an affected child must have two recessive alleles, so their genotype is aa. The child received one recessive allele from each parent. Since the parents are unaffected, neither can be aa. Each must have one dominant and one recessive allele, making both parents carriers. Homozygous recessive means having two recessive alleles, written aa. A carrier has one recessive allele but does not show the condition, written Aa.
How to calculate the chance for each child
Crossing two carrier genotypes gives four equally likely allele combinations. This gives each child a one-in-four chance of being affected. Each child's chance is calculated separately, so the result for one child does not determine the result for another. The four combinations are AA, Aa, Aa and aa. One in four children is expected to be affected, two in four to be unaffected carriers, and one in four to be unaffected and not a carrier. If a child is known to be unaffected, remove the affected aa possibility. Of the three remaining possibilities, two are carriers, so the chance of being a…
Use the pattern, not the count
A condition affecting most people in one family is not enough to prove that it is dominant. A small family can show an unusual proportion by chance. The specific pattern of two unaffected parents having an affected child is stronger evidence because it logically shows that both parents carried the recessive allele. An affected individual with a recessive condition must be homozygous recessive. The unaffected-parents, affected-child pattern identifies a recessive condition, regardless of how many other family members are affected.