A pedigree chart is a visual tool that tracks the inheritance of a specific trait through a family’s generations. It functions like a specialized family tree to show patterns of inheritance. The goal of analyzing a pedigree is to determine the genotype, which is the specific combination of genes an individual possesses for that trait. By reading these charts, it becomes possible to infer the genetic makeup of family members, which is useful for tracking genetic conditions and understanding heritability.
Fundamental Genetic Concepts for Pedigree Analysis
To interpret a pedigree, understanding genetic vocabulary is necessary. A gene is a segment of DNA that provides instructions for a trait, while alleles are the different versions of that gene. For example, a gene for eye color can have alleles for brown or blue eyes. An individual’s combination of alleles is their genotype, and the observable physical characteristic is their phenotype.
The relationship between alleles determines how a trait is expressed. An individual with two identical alleles for a gene is homozygous (AA or aa), while someone with two different alleles is heterozygous (Aa). A dominant allele is expressed in the phenotype if one copy is present (Aa), while a recessive allele is only expressed when two copies are present (aa).
Understanding Pedigree Chart Symbols and Structure
Pedigree charts use standardized symbols to represent a family’s structure. Males are depicted as squares and females as circles. A horizontal line connecting a square and a circle signifies a partnership, and a vertical line from this connection leads to their offspring. Siblings are connected by a horizontal line above them and are listed from left to right in order of birth.
Each generation is labeled with a Roman numeral (I, II, III), starting with the oldest at the top. Individuals within each generation are numbered with Arabic numerals (1, 2, 3). A shaded symbol indicates an “affected” individual who expresses the phenotype being studied, while an unshaded symbol represents an “unaffected” individual. A half-shaded symbol or a dot may denote a known heterozygous carrier, who carries a recessive allele but does not express the trait.
Key Genetic Inheritance Patterns
Analyzing a pedigree helps identify how a trait is inherited. In autosomal dominant inheritance, the trait is on a non-sex chromosome and appears in every generation. Every affected person has at least one affected parent, so the trait does not skip generations.
In autosomal recessive inheritance, the trait is also on a non-sex chromosome but can skip generations. Affected individuals can have unaffected parents who are both heterozygous carriers. This is often revealed when two unaffected parents have an affected child.
Inheritance can also be linked to sex chromosomes. In X-linked recessive traits, the gene is on the X chromosome, and males are more frequently affected. An affected mother will pass the trait to all of her sons. For X-linked dominant traits, affected fathers pass the trait to all of their daughters but none of their sons.
How to Determine Genotypes from Pedigrees
The first step is to analyze the pedigree to determine the mode of inheritance. A dominant pattern is suggested if the trait appears in every generation, while a recessive pattern is likely if it skips generations. Observing if the trait affects males and females at different rates helps distinguish between autosomal and sex-linked inheritance.
Once an inheritance pattern is established, you can assign genotypes. For a recessive trait, all affected individuals (shaded symbols) have a homozygous recessive genotype (aa). They must have inherited one ‘a’ allele from each parent, so if the parents are unaffected, their genotype must be heterozygous (Aa).
For individuals showing a dominant phenotype, their genotype could be homozygous dominant (AA) or heterozygous (Aa). To distinguish between these, examine their parents or offspring. If an individual with a dominant phenotype has a homozygous recessive (aa) parent or child, that individual must be heterozygous (Aa). When the exact genotype cannot be determined, it is noted with an underscore (e.g., A_) to show the dominant allele is present but the second allele is unknown.
Limitations in Pedigree Genotype Assignment
Pedigree analysis has limitations that can prevent the precise determination of every individual’s genotype. A common challenge is small family sizes. With few offspring, there may not be enough information to establish a clear inheritance pattern or to determine if an unaffected individual in a recessive pedigree is a heterozygous carrier or homozygous for the normal allele. The genotype of an unaffected parent may remain ambiguous.
In other instances, genetic complexities can obscure the analysis. A de novo mutation is a new genetic change that can cause a trait to appear in an individual without any family history. Another factor is incomplete penetrance, where an individual has the genotype for a trait but does not express the corresponding phenotype. This can make it appear as if a trait skips a generation, even in a dominant inheritance pattern, leading to misinterpretation.