Pedigree construction and inheritance patterns

Synopsis

Construct an accurate three-generation pedigree, recognise common Mendelian and non-Mendelian patterns, quantify uncertainty without overclaiming, and use the family history to guide genomic referral, testing and communication.

• Draw at least three generations when possible, using standard symbols, biological relationships, pregnancy outcomes, ancestry and consanguinity rather than relying on a narrative family list.
• Record each relevant diagnosis, age at onset, current age or age and cause of death, testing laboratory and whether documentary confirmation exists.
• Autosomal dominant disease often appears vertically in successive generations, affects all sexes and allows male-to-male transmission, although reduced penetrance can obscure the pattern.
• Autosomal recessive disease commonly affects siblings born to unaffected carrier parents; shared ancestry or consanguinity raises probability but is neither necessary nor diagnostic.
• X-linked conditions lack father-to-son transmission; expression in heterozygous females varies, so avoid calling every female relative an unaffected carrier from pedigree appearance alone.
• Mitochondrial variants are transmitted through the maternal line, but heteroplasmy and tissue distribution make severity and recurrence difficult to predict from one relative.
• A negative family history does not exclude genetic disease because of a de novo variant, small family, an unexpected biological relationship, adoption, early deaths, mosaicism, sex limitation or incomplete penetrance.
• Confirm relationships and consent before contacting relatives, separate reported from verified facts, and update the pedigree when diagnoses, pregnancies or genomic results change.