A pedigree is a family tree that tracks how a trait or disorder appears across generations. Biologists and genetic counselors use pedigrees to infer whether a trait is dominant, recessive, autosomal, or sex-linked. Reading a pedigree helps you connect visible patterns in families to the movement of alleles from parents to children.
This skill matters because it allows prediction of genetic risk even when not everyone in the family has been genetically tested.
Pedigrees use standard symbols, such as squares for males, circles for females, shaded shapes for affected individuals, and horizontal lines for mating pairs. The key strategy is to look for patterns across generations, such as affected children from unaffected parents or more males affected than females. Once the likely inheritance pattern is identified, genotypes can be assigned and Punnett square reasoning can be used to calculate probabilities.
A good pedigree analysis combines symbol reading, pattern recognition, and careful probability calculations.
Key Facts
- Square = male, circle = female, shaded symbol = affected individual, half-shaded symbol = carrier when shown.
- Autosomal dominant traits often appear in every generation and an affected child usually has at least one affected parent.
- Autosomal recessive traits can skip generations, and two unaffected carriers can have an affected child.
- For two heterozygous carriers of an autosomal recessive allele, Aa x Aa gives P(affected child) = 1/4.
- For an X-linked recessive trait, males are more often affected because males have only one X chromosome.
- Probability of two independent events both occurring = P(event 1) x P(event 2).
Vocabulary
- Pedigree
- A diagram that shows family relationships and the inheritance of a trait across generations.
- Autosomal
- A trait or gene located on one of the non-sex chromosomes.
- Dominant allele
- An allele that can produce a visible trait when only one copy is present.
- Recessive allele
- An allele that produces a visible trait only when two copies are present, unless it is on the single X chromosome in a male.
- Carrier
- An individual who has one recessive disease allele but does not show the recessive condition.
Common Mistakes to Avoid
- Assuming every shaded person must be homozygous is wrong because a person with an autosomal dominant trait is often heterozygous.
- Ignoring unaffected parents with affected children is wrong because this pattern is strong evidence for autosomal recessive inheritance.
- Treating X-linked traits exactly like autosomal traits is wrong because sons receive their X chromosome from their mother and their Y chromosome from their father.
- Forgetting to multiply independent probabilities is wrong because the chance of multiple children having specific genotypes must be calculated event by event.
Practice Questions
- 1 Two unaffected parents have an affected child with an autosomal recessive disorder. What are the most likely genotypes of the parents, and what is the probability their next child will be affected?
- 2 A woman is a carrier for an X-linked recessive disorder and her partner is unaffected. What is the probability that a son will be affected? What is the probability that a daughter will be a carrier?
- 3 A pedigree shows affected individuals in every generation, males and females affected equally, and every affected child has an affected parent. Which inheritance pattern is most likely, and what evidence supports your choice?