What is a Pedigree Chart?
A pedigree chart is a diagram that traces the inheritance of a specific trait or genetic condition through multiple generations of a family.
Pedigree charts are incredibly useful tools in genetics. They allow scientists, doctors, and genetic counsellors to:
- Track how a trait passes from parents to offspring
- Identify whether a condition is dominant or recessive
- Determine whether a trait is sex-linked or autosomal
- Predict the probability of future offspring inheriting a condition
Think of a pedigree chart as a family tree with extra information — instead of just showing who is related to whom, it also records who has a particular trait or condition.
Imagine you're trying to figure out why a particular eye colour keeps appearing in your family. A pedigree chart is like a detective's case board — it connects all the clues (family members and their traits) so you can figure out the pattern of inheritance.
Symbols Used in Pedigree Charts
Pedigree charts use a standardised set of symbols so that geneticists anywhere in the world can read them. Learning these symbols is the first essential step.
Shapes:
- Square (□) = Male individual
- Circle (○) = Female individual
- Diamond (◇) = Individual of unknown or unspecified sex
Fill (shading):
- Unfilled/white shape = Unaffected individual (does not show the trait)
- Filled/shaded shape = Affected individual (shows the trait)
- Half-shaded shape = Carrier (carries the allele but does not show the trait — used for recessive conditions)
Lines:
- Horizontal line connecting a square and circle = Mating pair (parents)
- Vertical line dropping down = Offspring line
- Horizontal sibship line = Connects siblings (brothers and sisters)
Other symbols:
- Roman numerals (I, II, III...) on the left = Generation number (I is the oldest)
- Arabic numerals (1, 2, 3...) = Individual number within a generation
When referring to an individual in a pedigree, you use their generation and number. For example, "II-3" means the third individual in the second generation.
11 more sections in this topic
Pick this up in your Library: it holds the whole topic, notes, cheatsheet and questions.