Constructing and interpreting genograms, particularly in relation to determining mode of inheritance

What a genogram is

A genogram/pedigree is a standardised diagrammatic representation of a family's structure and medical history across generations - the foundational tool for determining a condition's likely mode of inheritance and identifying at-risk relatives
Constructed using standardised symbols (squares for males, circles for females, filled symbols for affected individuals, lines denoting relationships) recognised internationally, allowing genograms to be interpreted consistently across clinicians and services

The pattern of affected individuals across generations reveals the likely inheritance pattern: autosomal dominant - affected individuals in multiple consecutive generations, both sexes equally affected, male-to-male transmission possible; autosomal recessive - typically only within one generation (siblings), often with parental consanguinity, both sexes equally affected; X-linked recessive - predominantly affects males, transmitted through unaffected/mildly affected carrier females, no male-to-male transmission; mitochondrial - transmitted only through the maternal line, affecting offspring of affected females but not offspring of affected males

At least three generations of family history should be documented where possible, including ages, cause of death, and specific diagnoses (not just "heart disease" or "cancer" - the specific type matters for inheritance pattern recognition)
Explicitly ask about consanguinity, pregnancy losses, and stillbirths, which are often not volunteered but are highly relevant to identifying a recessive or chromosomal cause

An accurately constructed genogram directly determines which relatives should be offered cascade testing/counselling once a pattern (or a specific pathogenic variant) is identified, and can itself suggest a likely inheritance pattern even before genetic testing results are available

Autosomal dominant, autosomal recessive, X-linked, and mitochondrial inheritance patterns (see Mendelian, sex-linked and mitochondrial inheritance), cascade testing of relatives

A genogram is not simply a family tree - its diagnostic value comes from careful, structured, specific questioning (exact diagnoses, ages of onset, consanguinity, pregnancy losses) that a cursory family history often misses entirely

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