A pedigree chart is used by geneticists and medical professionals to track the occurrence and inheritance of a specific trait or disorder across multiple generations of a family. The chart uses a standardized set of symbols to represent individuals and their biological relationships. By mapping out a family’s genetic history, researchers analyze the pattern of transmission to determine the underlying mode of inheritance for a given condition. This analysis helps in understanding the genetic basis of a trait and calculating the probability of its recurrence in future offspring.
Decoding the Standard Symbols
Understanding the basic geometric shapes and shading is necessary for interpreting a pedigree chart. The sex of an individual is represented by a specific shape: a square denotes a male, while a circle represents a female. A diamond shape is used when the sex of the individual is unknown.
The coloring or shading indicates the affected status for the trait being studied. A fully shaded symbol means that the person expresses the phenotype of the trait. Conversely, a completely unshaded symbol indicates that the individual is unaffected.
For recessive traits, a half-shaded shape or a shape containing a central dot represents a carrier, or a heterozygous individual. This person does not express the trait but carries one copy of the gene variant and can pass it on. A diagonal line drawn through any symbol indicates that the person is deceased. An arrow pointing to a specific symbol designates the proband, the first family member identified with the trait and who brought the family to genetic attention.
Mapping Generations and Relationships
A pedigree chart organizes individuals into distinct generations. Generations are consistently labeled with Roman numerals (I, II, and III), starting with the oldest generation at the top of the chart. Within each generation, individuals are numbered sequentially from left to right using standard Arabic numerals, enabling precise identification of any family member, such as II-3.
Relationships between individuals are defined by a series of horizontal and vertical lines connecting the symbols. A horizontal line drawn between two symbols of opposite sex signifies a mating or partnership. From this mating line, a single vertical line descends to a second horizontal line, known as the sibship line.
The sibship line connects all the offspring from that partnership, with children typically arranged in birth order. Special circumstances have their own conventions. A dashed line of descent indicates an adopted child, or diagonal lines branching from the sibship line represent twins. Monozygotic (identical) twins are distinguished by a horizontal bar connecting their diagonal lines, whereas dizygotic (fraternal) twins lack this connecting bar.
Analyzing Inheritance Patterns
The pattern of trait transmission is analyzed to determine its genetic nature. A quick, initial assessment can distinguish between dominant and recessive traits by looking for a pattern of skipping generations. If the trait appears in every generation, it is likely a dominant trait, as an individual needs only one copy of the gene variant to be affected.
Autosomal Traits
A rule for identifying an autosomal dominant trait is that every affected individual must have at least one affected parent. Furthermore, if two unaffected parents have an affected child, the trait cannot be dominant, as two parents without the dominant gene variant cannot produce an offspring with it.
Autosomal recessive traits, conversely, are often characterized by the appearance of the trait in offspring whose parents are both unaffected. This “skipping” occurs because the unaffected parents are carriers of the recessive gene variant, and the affected child inherits one copy from each.
X-Linked Traits
To differentiate autosomal traits, which involve the non-sex chromosomes, from X-linked traits, which are carried on the X chromosome, specific transmission patterns involving sex must be examined. A rule for excluding X-linked inheritance is the observation of an affected father passing the trait to an affected son. This is impossible for X-linked traits because a male inherits his X chromosome from his mother and his Y chromosome from his father.
For X-linked dominant traits, an affected father will pass the trait to all of his daughters, as all daughters inherit his single X chromosome, but none of his sons. In X-linked recessive inheritance, males are affected far more frequently than females, as males only have one X chromosome, meaning a single recessive gene variant is sufficient for the trait to be expressed. Another indicator for X-linked recessive traits is that all sons of an affected mother must also be affected, since she only has affected X chromosomes to pass on.