Legal and ethical principles of genetic testing - familial implications of a genetic diagnosis, including discussion of autosomal recessive, autosomal dominant, and X-linked inheritance

The family diagnosis

A genetic diagnosis in one individual carries direct implications for their biological relatives, who may share the same variant and its associated risk - the inheritance pattern (autosomal recessive, autosomal dominant, or other) determines exactly which relatives are at risk and by how much
Every confirmed pathogenic genetic diagnosis creates a "family" dimension to the case, not just an individual one - cascade testing of relatives is a core, expected part of clinical genetics practice, not an optional extra

Autosomal dominant - each first-degree relative (parent, sibling, child) has a 50% risk of carrying the same variant, regardless of sex
Autosomal recessive - siblings of an affected individual have a 25% risk of being affected (and 50% chance of being an unaffected carrier); parents are obligate carriers; the general population risk to more distant relatives is much lower unless consanguinity is present
Distinguishing the two patterns correctly is essential, since the risk communicated to relatives, and who specifically should be offered testing, differs substantially between them

Cascade testing - systematically offering testing to at-risk relatives once a familial pathogenic variant is identified - is the standard approach to translate an individual diagnosis into family-wide risk information and, where relevant, preventive action
Constructing an accurate pedigree/genogram is the essential first step to correctly identify which relatives are at risk under the specific inheritance pattern involved

The proband (the family member through whom the family was ascertained, generally the first person diagnosed) has an ethical and often practical responsibility to facilitate information reaching at-risk relatives, though direct disclosure obligations to relatives (rather than through the patient) are legally and ethically contested and vary by jurisdiction
Family communication can be practically and emotionally difficult - genetic counsellors specifically support this process, including via family letters relatives can share, rather than leaving it solely to the patient without support

Cascade genetic testing, familial hypercholesterolaemia (a well-established cascade testing model), hereditary cancer syndromes, genetic counselling

The clinical benefit of a genetic diagnosis is often realised as much (or more) through cascade testing of relatives - identifying and intervening in at-risk family members before they develop symptoms - as through management of the original proband alone

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