Why Pedigree Notation Matters
A pedigree is a compact diagram of recorded family relationships. Before using one to organize genetic information, a reader must be able to tell which symbol represents which person and how the connecting lines indicate parentage. A careful reading also separates what the diagram explicitly records from what it does not establish: a pedigree can show a parent–offspring link, for example, without showing anyone’s genotype.
The conventions below describe common traditional pedigree notation. Symbols and line styles can vary across diagrams, so a supplied legend takes precedence. In every case, interpret the relationship from the diagram’s connections rather than from a person’s position alone.
Symbols for Individuals
In traditional pedigree notation, a square usually represents a male and a circle usually represents a female. A diamond is often used when sex is unspecified or not shown. These shapes identify individuals; they do not, by themselves, indicate whether a person has a trait or carries a genetic variant.
Shading commonly adds information about a recorded trait or status. A fully shaded symbol often denotes an individual with the trait being tracked, while an unshaded symbol often denotes an individual without it. Partial shading or other marks may indicate a carrier or another status, but those meanings are not universal. Always check the figure’s legend before interpreting fill, color, or marks inside a symbol. A diagonal slash through a symbol commonly indicates that the individual is deceased; it does not change the person’s position in the pedigree.
| Diagram feature | Common interpretation | What not to assume |
|---|---|---|
| Square | Male individual in traditional notation | That the individual has a particular genotype |
| Circle | Female individual in traditional notation | That the individual is unaffected |
| Diamond | Sex unspecified or not represented | A particular sex or relationship not given by the diagram |
| Shading | Status for the trait or feature defined by the legend | A universal meaning independent of the legend |
| Slash through symbol | Individual is deceased in common notation | That the person is absent from the family relationships shown |
Lines Show Relationships
In a simple pedigree, a horizontal line joining two individuals commonly represents a partner or mating relationship. A line descending from that pair leads to their child or to a horizontal sibship line from which the children’s individual lines descend. Children connected to the same sibship line are shown as siblings. Thus, the useful reading direction is from the parents’ connection downward to the child symbols, not from an individual’s left-to-right position.
A child’s connection to a pair identifies the parents recorded in that pedigree. It does not necessarily tell the reader whether the relationship is genetic, adoptive, or otherwise defined; the diagram’s conventions or accompanying description must settle that. A broken or dashed line may have a special meaning, such as adoption, in a particular source, but no single interpretation should be assumed without a legend.
A line that crosses another line is not automatically a relationship junction. In a clear pedigree, a connection is indicated by an actual join or branching point. If it is unclear whether two lines meet, the figure may be ambiguous and should not be resolved by guessing. Likewise, being drawn next to someone does not make two individuals siblings or partners; the connecting lines carry that information.
Worked Example: Reading a Couple and Their Children
Consider a hypothetical diagram with square I-1 and circle I-2 joined by a horizontal line. A vertical line descends from their connection to a sibship line. Three vertical branches from that sibship line lead to circles II-1 and II-3 and square II-2. Assume the numbering proceeds from left to right within each generation. Identify the parents, children, and sibling relationships.
Parents: I-1 and I-2 are the pair joined in generation I. The descent line from their connection leads to the sibship line, so the three individuals below it are their children in the relationships recorded by this pedigree.
Children: The three child symbols are II-1, II-2, and II-3. Each is in generation II because each is one parent–offspring step below the generation I pair.
Sibling relationships: Since all three branches descend from the same sibship line, II-1, II-2, and II-3 are shown as siblings. II-1 and II-3 are represented as female, while II-2 is represented as male under the traditional symbol convention. No conclusion about their trait status or genotypes follows because no such information is specified.
Generations and Individual Labels
Generations are commonly labeled with Roman numerals: I for the first generation shown, II for the next, and III for the one after that. Individuals within a generation are commonly numbered with Arabic numerals, usually from left to right. A combined label such as II-3 therefore means the third numbered individual in generation II of that pedigree. The label is an identifier, not a statement about birth order unless the diagram’s numbering convention says so.
Numbering is local to a pedigree. “II-3” in one diagram is not automatically the same person as “II-3” in another. A numbering scheme also does not imply that every person in a family is included. The pedigree may show only the relatives relevant to the question or the available records.
Worked Example: Assigning Generation Labels
In a hypothetical pedigree, two individuals form the top pair. They have two children, and one of those children later has a child of their own. The diagram numbers individuals from left to right within each generation. Determine the generation of each person and explain why the grandchild is not assigned to generation II.
The top pair is generation I. Both children are one parent–offspring step below that pair, so both are in generation II. The child of one generation II individual is one further parent–offspring step below the original pair, so that individual is in generation III. The number after the Roman numeral distinguishes the person within the relevant level; it does not change the generation.
The logic can be stated directly. If \(g(P)\) is the generation number assigned to a parent \(P\), the convention requires \(g(C)=g(P)+1\) for a child \(C\). Along two successive parent–offspring links, \(g(G)=g(P)+2\), where \(G\) is the child’s child. Therefore, the grandchild belongs to generation III when the top pair is generation I, not generation II.
Tracing Parentage and Shared Ancestors
To identify a person’s parents, trace upward from that person’s symbol along the descent branch to the sibship line and then to the parent connection. To identify siblings, check whether their branches arise from the same sibship line. To find a shared ancestor for two people, trace each person’s parentage upward and look for an individual or pair appearing on both paths. A shared ancestor is identified by the paths in the diagram; it is not inferred merely because the two people are drawn close together.
A diagram may show a shared ancestor without showing every connecting relative in detail. If one person’s parentage is unknown or a branch is omitted, the available information may not establish whether two individuals share an ancestor. State what the pedigree supports and leave unresolved relationships unresolved. Identifying a shared ancestor is a structural reading task; it does not by itself quantify genetic sharing.
Worked Example: Finding a Shared Ancestor
Consider a hypothetical three-generation pedigree. In generation I, square I-1 and circle I-2 have two children, II-1 and II-2. Each of those generation II individuals has one child: II-1 has III-1, and II-2 has III-2. The diagram uses the same descent and sibship-line conventions described above. Determine the relationship supported by the diagram between III-1 and III-2, and identify their shared ancestors.
Trace upward from III-1: the diagram connects this person to II-1, whose parents are I-1 and I-2. Trace upward from III-2: this person connects to II-2, whose parents are also I-1 and I-2. Thus, III-1 and III-2 are shown as children of siblings II-1 and II-2, and they share the two generation I individuals, I-1 and I-2, as ancestors.
This conclusion follows from the displayed parentage links, not from the positions of the two generation III symbols. It does not require or justify a numerical statement about genetic relatedness; the pedigree-reading task here is to name the paths and shared ancestors.
A Systematic Reading Method
A short sequence of checks prevents many misreadings, especially when a pedigree contains several branches. First inspect the legend for symbol and line meanings. Next identify the generation labels and the individual identifiers. Then follow each relevant child branch upward to its recorded parent connection. Finally, report only the relationships and shared ancestors supported by those connections.
Confirm what shape, shading, marks, and line styles mean in this particular diagram.
Use the generation label and individual number to name each person unambiguously.
Follow descent lines to parent connections and check which branches share a sibship line.
Name the parent–offspring links or shared ancestors shown, and note any relationships the diagram leaves unspecified.
Common Mistakes and Why the Notation Matters
Pedigrees are often used as the structural record of a family in genetic analyses. If a parent–offspring link is misread, individuals may be assigned to the wrong branch or a shared ancestor may be missed. Accurate reading therefore comes before any calculation that uses family structure. The symbols organize information, but they should not be treated as evidence for information they do not encode.
- Wrong reasoning: “The person is shaded, so the symbol must mean they carry a particular allele.” Correction: Shading has the meaning given by the legend, often a recorded trait status; it does not reveal a specific genotype unless that is explicitly defined.
- Wrong reasoning: “These two symbols are close together, so they are siblings.” Correction: Sibling status is indicated by their descent from the same sibship line, not by visual proximity.
- Wrong reasoning: “The third symbol drawn is the third-born child.” Correction: Individual numbers commonly follow left-to-right order, but birth order should not be claimed unless the diagram establishes that convention.
- Wrong reasoning: “A dashed line always means adoption.” Correction: Line styles can be diagram-specific. Check the legend or description before assigning a meaning.
- Wrong reasoning: “A pedigree proves the biological parentage shown.” Correction: It records the relationships represented under its conventions; the diagram alone does not validate how those relationships were established.
Check Your Understanding
Answer each question using the pedigree conventions and limits described above.
- In traditional notation, what do a square and a circle commonly represent, and what information do their shapes not provide?
- Two children descend from the same sibship line. What relationship does the diagram show between them?
- A person in generation II has a child. Which generation contains that child, and what rule determines the answer?
- Two people trace upward to the same pair of generation I individuals. What can you conclude about their shared ancestors?
- A pedigree uses dashed parentage lines but provides no legend. Why should you avoid assigning a specific meaning to the dashed line?