Inheritance pattern identifier: reading mode of inheritance from a pedigree

Short version. Inheritance pattern identification is probabilistic, not deterministic. No small pedigree uniquely determines mode of inheritance; most patterns are compatible with several inheritance models, and molecular confirmation is ultimately necessary. The practical workflow is: (1) check the discriminating features of each pattern, (2) apply the exclusion rules (male-to-male transmission excludes X-linked; affected fathers transmitting to none of their children for a given condition hints at Y-linked or mitochondrial), (3) consider the pitfalls (reduced penetrance, new mutations, pseudodominance, phenocopies, ascertainment bias), and (4) weight the differential by plausibility of each pattern given the observed pedigree.

The six classical patterns at a glance

Autosomal dominant — red flags and traps

Red flags: the condition appears in successive generations; affected individuals have an affected parent; roughly half of the offspring of affected parents are affected; male-to-male transmission is present; both sexes are affected in similar numbers.

Traps: reduced penetrance creates apparent skipping, making AD look recessive; de novo mutations create sporadic cases in families with no prior history; ascertainment bias inflates observed transmission ratios; variable expressivity causes mild cases to be missed on pedigree coding. Classic examples: Huntington disease, neurofibromatosis type 1, Marfan syndrome, familial hypercholesterolaemia, HBOC (BRCA1/BRCA2). See our autosomal dominant calculator for the calculation detail.

Autosomal recessive — red flags and traps

Red flags: affected siblings with unaffected parents; horizontal pattern within a sibship; both sexes affected similarly; presence of consanguinity; ancestry from a population with elevated carrier frequency for the candidate condition.

Traps: pseudodominance from a homozygous affected partnering a carrier, making an AR pattern look dominant; genetic heterogeneity where similar phenotypes are caused by different genes (so no single gene pattern describes the full family); small sibships where the expected 1/4 ratio is noisy; phenocopies where an unrelated environmental cause produces similar phenotype. Classic examples: cystic fibrosis, sickle cell disease, spinal muscular atrophy, Tay-Sachs disease, phenylketonuria. See our autosomal recessive calculator.

X-linked recessive — red flags and traps

Red flags: predominantly affected males; a carrier mother transmitting to approximately half her sons; no male-to-male transmission; all daughters of an affected father are obligate carriers; the condition appears to skip through carrier mothers.

Traps: skewed X-inactivation creating mildly affected carrier females, which can lead to recategorising a "carrier" as "affected" and muddling the pattern; new mutations at conditions like DMD, where the mother of an isolated case is not automatically a carrier; homozygous affected females in high-frequency conditions or consanguineous families, which can masquerade as autosomal inheritance. Classic examples: haemophilia A and B, Duchenne muscular dystrophy, G6PD deficiency, red-green colour blindness. See our X-linked recessive calculator.

X-linked dominant — red flags and traps

X-linked dominant (XD) conditions affect heterozygous females as well as hemizygous males. An affected father transmits to all of his daughters (all affected) and to none of his sons. An affected heterozygous mother transmits to 50% of her children of either sex.

Red flags: affected males and females in similar frequencies but with females sometimes less severely affected; all daughters of affected fathers are affected; no male-to-male transmission; the condition often appears in every generation.

Traps: some XD conditions are male-lethal (for example, incontinentia pigmenti, Rett syndrome in many cases), producing pedigrees with recurrent miscarriages of male fetuses and only affected females. Classic examples: X-linked hypophosphataemic rickets, fragile X (with considerable complexity), incontinentia pigmenti.

Mitochondrial (maternal) inheritance — red flags and traps

Mitochondrial DNA is inherited exclusively from the mother. An affected mother transmits the mitochondrial variant to all of her children, though the proportion of affected mitochondria (heteroplasmy) and the clinical expression varies. An affected father never transmits the condition.

Red flags: transmission only through the maternal line; both sexes affected among the offspring of affected mothers; highly variable severity due to heteroplasmy and tissue-specific mitochondrial burden.

Traps: nuclear genes affecting mitochondrial function produce autosomal (not mitochondrial) inheritance patterns while still presenting as "mitochondrial disease" clinically; heteroplasmy causes substantial expressivity variation that can look like incomplete penetrance; tissue-specific manifestation (for example, Leber hereditary optic neuropathy) may look sex-biased because of tissue-level threshold effects. Classic examples: Leber hereditary optic neuropathy, MELAS, MERRF.

Y-linked — red flags and traps

Y-linked inheritance is limited to genes on the non-recombining portion of the Y chromosome. An affected father transmits to all of his sons and to none of his daughters. Females are never affected.

Red flags: exclusively male transmission and male-only manifestation. Few clinically significant Mendelian conditions are purely Y-linked; the most widely cited examples relate to Y-linked spermatogenic failure from AZF deletions, typically identified through infertility workups rather than traditional pedigrees.

The pitfall list — always check these

Before committing to a pattern, audit the pedigree for the following:

• Reduced penetrance: a dominant condition can skip generations when unaffected carriers transmit the variant.
• New (de novo) mutations: a dominant or X-linked condition can appear sporadic if the proband's mutation arose de novo.
• Parental germline mosaicism: two or more affected children born to unaffected parents with an AD or X-linked dominant condition is a red flag for parental germline mosaicism — one parent's gametes carry the variant in some fraction without the variant being detectable in blood. Recurrence risk is substantially elevated above the de novo baseline. See the germline mosaicism calculator for the posterior calculation with optional somatic VAF.
• Pseudodominance: an autosomal recessive condition can look dominant when an affected homozygote partners a carrier.
• Phenocopies: an unrelated environmental or polygenic cause can mimic the Mendelian condition in one family member.
• Genetic heterogeneity: similar phenotypes from different genes can confuse pattern recognition across a mixed family.
• Consanguinity: increases recessive-disease probability and the prior for pseudodominance.
• Non-paternity or adoption: invalidates assumed biological relationships.
• Ascertainment bias: families come to attention because multiple members are affected, inflating observed transmission.
• Small sibships: expected ratios are noisy; do not overfit.
• Parental age: higher paternal age increases de novo mutation rate for some conditions.

A two-pass heuristic

A useful default workflow:

1. First pass — exclusion. If male-to-male transmission is present, exclude X-linked patterns. If only the maternal line transmits to all offspring, consider mitochondrial. If only fathers transmit to only sons, consider Y-linked.
2. Second pass — match. Between the remaining candidates, choose the best fit: vertical-through-every-generation with ~50% transmission suggests AD; horizontal within a sibship with unaffected parents suggests AR; predominantly affected males through carrier females suggests XR.
3. Third pass — audit for pitfalls. Consider reduced penetrance, new mutations, and ancestry before committing.

How Evagene supports this

Evagene evaluates a drawn pedigree against all six classical inheritance patterns and scores the consistency of each, based on pedigree features such as vertical/horizontal pattern, sex distribution, presence or absence of male-to-male transmission, and the location of affected individuals relative to obligate carriers. When molecular test results or a specific catalogued disease (ICD-10/OMIM) are annotated, the calculator uses the expected inheritance pattern of that condition as a strong prior while still checking for consistency with the observed family.

AI-assisted interpretation, running through the Mendelian inheritance module with your own Anthropic or OpenAI LLM key, then drafts a structured narrative: identifies the likely inheritance pattern, names the obligate carriers and at-risk individuals, flags pitfalls that are worth considering in this specific pedigree (reduced penetrance, small sibships, suspected pseudodominance, and so on), and recommends next steps such as targeted molecular testing or cascade carrier screening. Interpretation is a drafting aid for the clinician's report, not a substitute for clinical judgement.

Where cancer risk is in play, BRCAPRO, MMRpro, and PancPRO provide empirical risk estimates that complement the pattern-level reasoning.

Frequently asked questions

Related reading

• Mendelian inheritance calculator overview
• Autosomal dominant calculator
• Autosomal recessive calculator
• X-linked recessive calculator
• Carrier probability calculator
• Consanguinity calculator
• Clinical genetics pedigree tools