Pedigree Analysis: Tracing Traits in Family Trees

A pedigree is a diagram of a family tree that records who has a particular trait across several generations. Pedigree analysis means reading that diagram to work out how the trait is inherited — whether it is dominant or recessive, and whether its gene is on an autosome or a sex chromosome.

This is a key skill wherever families carry genetic diseases. Since we cannot do test crosses on people, pedigrees are how doctors predict risks for future children and decide who should be offered genetic testing.

Key Concepts

• Square: a male; circle: a female; a filled symbol means affected, an open symbol means unaffected
• Horizontal line between partners: mating; vertical lines lead down to their children, oldest on the left
• Proband: the person who first brought the family to medical attention
• Generations: labelled with Roman numerals (I, II, III) down the side
• Obligate carrier: a person who must carry the allele, judged by their children or parents

How It Works

Step 1: Ask whether the trait skips generations. If two unaffected parents have an affected child, the trait must be recessive. Step 2: If an affected person appears in every generation and affected parents can have unaffected children, the trait is likely dominant. Step 3: To test for sex-linkage, check who is affected. If mostly males are affected and it never passes father-to-son, it is probably X-linked recessive. If affected fathers pass it to all daughters, consider X-linked dominant. Step 4: Assign genotypes (A/a or XA/Xa) and calculate risks for the next child.

Worked Example

In a family, unaffected parents (I-1 and I-2) have three children: a normal son, an affected daughter, and a normal daughter.

Step 1: Both parents are healthy but have an affected child, so the disorder is recessive.

Step 2: The affected child is a daughter. Her father is normal but she received an allele from him too — if it were X-linked recessive, the father would have to be affected to give her a mutant X. He is not, so it is autosomal recessive.

Step 3: Both parents must be carriers (Aa × Aa). Each future child has a 25% chance of being affected (aa), regardless of sex.

Common Misconceptions

• Misconception: If a trait is common in a family, it must be dominant. Correction: Common recessive traits (like blood group O families) can appear often; the key test is whether unaffected parents have affected children.
• Misconception: A filled symbol means the person carries the gene. Correction: Filled means actually affected; carriers of recessive traits look normal and stay unfilled unless the diagram marks carriers specially.
• Misconception: Pedigrees predict a child with certainty. Correction: They give probabilities — a 25% risk applies fresh to every child.

Real-World Applications

Genetic counsellors draw pedigrees for families with cystic fibrosis, thalassemia or Huntington's disease to guide testing and pregnancy decisions. Hospitals use them to spot members who should start early screening, such as for inherited cancers. Researchers studying rare diseases collect large pedigrees to locate the gene. Livestock breeders and dog registries also keep pedigree records to avoid mating close relatives or known carriers.

A pedigree turns a family's memory into data — from a grid of squares and circles, the hidden pattern of a gene reveals itself.