A pedigree is a visual chart illustrating a family’s lineage and how specific traits or conditions pass through generations. It maps relationships among family members and indicates who expresses a particular characteristic. This representation helps recognize inheritance patterns over time.
The Purpose of Pedigrees
Pedigrees help understand genetic inheritance within families. They allow researchers and healthcare professionals to track inherited traits or genetic disorders across multiple generations. By mapping these characteristics, pedigrees identify specific inheritance patterns, such as autosomal dominant, recessive, or X-linked traits. This organization aids in assessing genetic risk and provides valuable insights for genetic counseling and medical diagnoses.
Essential Pedigree Symbols
Constructing a pedigree relies on a standardized set of symbols to represent individuals and their relationships.
A square denotes a male; a circle represents a female. A diamond is used for individuals of unspecified sex.
Shading or coloring a symbol indicates an individual is affected by the trait. An unshaded symbol signifies an unaffected individual.
A half-shaded symbol, or a dot, denotes a carrier, meaning they possess one copy of a gene for a recessive trait but do not express it.
A diagonal line through a symbol indicates a deceased individual.
Horizontal lines connect individuals who are mating. A vertical line extending downwards from this mating line connects to their offspring, arranged in birth order from left to right.
Double horizontal lines between parents indicate a consanguineous marriage, a union between closely related individuals.
A diagonal line through the mating line signifies a divorce or separation.
How to Construct a Pedigree
Creating an accurate pedigree begins with thorough information gathering from family members. Identify the “proband,” the individual through whom the family came to attention, and gather health information about their siblings, parents, and grandparents. Collect details on birth order, gender, relevant health conditions, and whether individuals are affected or unaffected by the trait of interest.
Once information is collected, organize it by generation, starting with the oldest at the top. Use Roman numerals (I, II, III, etc.) to label each generation. Within each generation, assign Arabic numerals (1, 2, 3, etc.) to individuals, numbered from left to right based on birth order.
Begin drawing the pedigree using standard symbols. Connect parents with a horizontal mating line, drawing a vertical line down to their children. Siblings are connected by a horizontal line above them, stemming from the same vertical line from their parents. Ensure all symbols are correctly drawn and shaded according to the individual’s status. Label each individual with their Arabic numeral and their generation with its Roman numeral.
Analyzing Your Pedigree
After constructing a pedigree, the next step involves interpreting visual patterns to gain insights into the trait’s inheritance. Observing the distribution of affected individuals across generations can suggest different modes of inheritance. If the trait appears in every generation and affects both males and females, it often indicates an autosomal dominant pattern. Conversely, if the trait skips generations and primarily affects individuals whose parents are unaffected carriers, it may suggest an autosomal recessive pattern.
The presence of a trait predominantly in males, with affected sons born to unaffected mothers, can point to X-linked recessive inheritance. By tracing the trait’s appearance and absence through the family tree, one can deduce the likelihood of an individual being a carrier or affected. This analysis helps in predicting the probability of the trait appearing in future offspring. This understanding allows for informed genetic counseling and risk assessment for family members.