Inheritance - Mendelian

Definition

Mendelian (single-gene) inheritance describes conditions caused by a variant in a single gene, following predictable transmission patterns - autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive - the foundational patterns underlying most classic genetic disease counselling
The majority of individually rare, well-characterised genetic disorders follow a Mendelian pattern, in contrast to common complex diseases (diabetes, most cancers, cardiovascular disease), which are multifactorial/polygenic

Autosomal dominant - one pathogenic copy is sufficient to cause disease; 50% risk to offspring of an affected parent; both sexes affected equally
Autosomal recessive - two pathogenic copies (one from each parent) needed; carrier parents are typically unaffected; 25% risk to offspring of two carrier parents; both sexes affected equally
X-linked recessive - predominantly affects males (who have only one X); female carriers are typically unaffected (though can be mildly affected via skewed X-inactivation); no male-to-male transmission
X-linked dominant - affects both sexes, though can be more severe or even lethal in males (who lack a second, potentially compensating X)

Pedigree analysis (see genograms) is the primary tool for recognising the likely Mendelian pattern before genetic testing confirms the specific gene/variant
Recurrence risk counselling follows directly and predictably from the established pattern once confirmed, in contrast to the more probabilistic risk estimates required for multifactorial/polygenic conditions
Genetic counselling communicates recurrence risk based on the confirmed pattern, and cascade testing targets the specific at-risk relatives implied by that pattern (all first-degree relatives for autosomal dominant; siblings specifically for autosomal recessive; maternal relatives for X-linked)

Cystic fibrosis (autosomal recessive), Huntington disease (autosomal dominant), haemophilia A/B (X-linked recessive), each covered in more depth in their own disease-specific notes

Real-world inheritance is not always this clean - reduced penetrance, variable expressivity, and genetic/locus heterogeneity (the same clinical phenotype caused by variants in different genes) can all complicate the apparently straightforward Mendelian pattern seen in a family

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