How do family pedigrees show autosomal dominant inheritance?

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BRCA2 tumor suppressor gene on chromosome 13 — human genetic disease Photomicrograph of the *BRCA2* tumor suppressor gene on chromosome 13 of the human genome. Inactivation of this growth-regulating gene is associated with a higher risk of developing breast cancer.

In the study of human genetic disease, a pedigree—a pictorial representation of family history—demonstrates autosomal dominant inheritance through a "vertical" pattern, meaning the trait is typically passed from one generation to the next.1

Key features of these pedigrees include:

Vertical Transmission: Affected individuals appear in multiple successive generations.1
Symbolism: Circles represent females and squares represent males. Solid symbols denote affected individuals, while open symbols represent those who are unaffected.1
Mating and Siblings: Mating is indicated by two symbols joined at the midpoint, while siblings are suspended from a common overhead line, ordered by birth from left to right.1
Identification: Generations are marked with Roman numerals, and individuals within those generations are identified by Arabic numerals (e.g., II-1).1

While these patterns are characteristic, autosomal dominant traits can occasionally arise from new mutations in unaffected parents or involve "nonpenetrant" individuals who carry the gene but show no symptoms, yet can still pass the condition to their offspring.1