Expressivity (genetics) in the context of "Proband"

In medical genetics and other medical fields, a proband, propositus (male proband), or proposita (female proband) is a particular subject (human or other animal) being studied or reported on. On pedigrees, the proband is noted with a triangle (male) or circle (female) shaded accordingly. Denoting the proband is important, so the relationship to other individuals can be seen and patterns established.

In most cases, the proband is the first affected family member who seeks medical attention for a genetic disorder. Among the ancestors of the proband, other subjects may manifest the disease, but the proband typically refers to the member seeking medical attention or being studied, even if affected ancestors are known. Often, affected ancestors are unknown due to the lack of information regarding those individuals or about the disease at the time they lived. Other ancestors might be undiagnosed due to the incomplete penetrance or variable expressivity.