01Core principlesThe concepts and mechanisms needed to understand the subject.
Human genetics links variation in DNA and chromosomes to biological traits and disease. A genotype is the sequence or structural state; phenotype is the observed result shaped by genes, development, environment, chance and measurement. Penetrance is the proportion of variant carriers who express a defined phenotype by a defined age, while expressivity describes variation among those affected. Pleiotropy means one genetic mechanism affects several systems; locus heterogeneity means different genes can produce a similar phenotype.
Mendelian probabilities begin with meiosis, but the clinical question must specify whose genotype is known. An affected heterozygous parent with a fully penetrant autosomal-dominant condition transmits the variant to half of children of either sex. If both parents carry the same autosomal-recessive disease allele, each pregnancy has 25% affected, 50% carrier and 25% neither-variant probability. These outcomes reset for each conception. X-linked probabilities depend on the sex chromosomes transmitted; mitochondrial DNA is usually transmitted through the egg, although heteroplasmy and bottleneck effects complicate severity.
Genomic tests sample different variant classes. Karyotyping detects large chromosomal changes; chromosomal microarray detects submicroscopic copy-number imbalance; targeted sequencing examines specified genes; exome and genome analysis broaden scope but do not uniformly capture every repeat, structural, methylation or low-level mosaic change. Analytical sensitivity asks whether the method detects a variant class. Clinical sensitivity asks how often a relevant test finds an explanatory result in people with the phenotype. A negative report can therefore leave substantial residual genetic probability.
Interpretation is an evidence synthesis. Population frequency, predicted molecular consequence, functional data, segregation, de novo status, allelic observations and phenotype match contribute to classification. Categories range from benign to pathogenic, with uncertainty between. Classification can change as evidence accumulates. Even a pathogenic variant may be incidental or insufficient to explain the whole presentation. Consent and post-test communication must address implications for relatives without assuming that one person's result can be disclosed casually or that relatives' choices are identical.
Key points
- Separate genotype, phenotype, penetrance and variable expression: inheriting a pathogenic variant may not predict whether, when or how severely disease appears.
- Autosomal dominant, autosomal recessive, X-linked and mitochondrial patterns generate probabilities per pregnancy, conditional on the parental genotypes and variant mechanism.
- A negative result excludes only variants the chosen assay could detect in genes and regions relevant to the tested indication.
- Sequence variants, copy-number changes, repeat expansions, methylation abnormalities and mosaicism may require different laboratory methods.
- Classify germline variants using multiple evidence types; pathogenicity is distinct from whether a variant explains this patient's phenotype.
- A variant of uncertain significance is not a basis for predictive testing or changing clinical management; segregation or new evidence may permit later reclassification.
- Before testing discuss possible results, uncertainty, family implications, data use and choices; after testing communicate residual risk and a clear plan.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Affected people in successive generations and male-to-male transmission support autosomal dominance, while reduced penetrance can create an apparently skipped generation.
Affected siblings with unaffected parents, consanguinity or a founder ancestry can support recessive inheritance, but small families may conceal the pattern.
Absence of male-to-male transmission and sex-dependent severity can suggest X linkage; skewed X inactivation may make heterozygous females symptomatic.
Transmission through affected mothers but not affected fathers suggests mitochondrial inheritance, while heteroplasmy produces tissue and sibling variability.
A post-zygotic variant may be absent or low-level in blood, produce segmental features and create recurrence risk that depends on gonadal involvement.
A result that poorly fits age, organ pattern or inheritance may be incidental, incomplete or incorrectly framed and requires laboratory-clinical review.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Three-generation pedigree - Why
- Represent affected status, ancestry, reproductive history and biological relationships across a family.
- Interpretation and limitations
- Record ages and uncertainty, verify diagnoses where possible and avoid inferring inheritance from a small or incomplete family alone.
- 02
Phenotype specification - Why
- Give the laboratory discriminating clinical features and plausible disease mechanism.
- Interpretation and limitations
- Use precise features, onset and relevant negatives; an overly broad label lowers interpretive power and can increase uncertain findings.
- 03
National Test Directory selection - Why
- Match an NHS England-commissioned test and eligibility criteria to the clinical indication.
- Interpretation and limitations
- Confirm the current directory version, required specialist route and sample set; eligibility and technology can change between releases. Elsewhere in the UK, use the applicable national or regional genomics service pathway.
- 04
Variant-class coverage review - Why
- Determine whether the assay can detect the suspected sequence, copy-number, repeat, methylation or mosaic mechanism.
- Interpretation and limitations
- Read limitations and coverage rather than treating exome or genome as universal; a negative assay does not exclude variants outside its validated scope.
- 05
Variant classification and phenotype correlation - Why
- Evaluate pathogenic evidence and whether the finding explains this person.
- Interpretation and limitations
- Use the laboratory category and supporting evidence, then assess inheritance and phenotype separately; do not promote a VUS to causative from plausibility alone.
- 06
Segregation or targeted family testing - Why
- Clarify inheritance, phase or variant relevance and inform relatives where appropriate.
- Interpretation and limitations
- Test the most informative relatives through an agreed genetics plan; non-segregation may weaken causality but penetrance, phenocopies and mosaicism complicate interpretation.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked exampleCalculate autosomal-recessive recurrence riskInputs: both parents are confirmed heterozygous carriers of the same pathogenic recessive variant; their first child is affected; calculate outcomes for the next pregnancy.+
- 1Represent each parent as A/a, where a is the disease-associated allele; each produces A and a gametes with probability one half.
- 2Combine independent gametes: AA probability one quarter, A/a plus a/A total probability one half, and a/a probability one quarter.
- 3State the next-pregnancy outcomes: 25% affected, 50% unaffected carrier and 25% unaffected non-carrier, assuming the model and parentage are correct.
- 4Explain that the affected first child does not change the next conception's probabilities because each meiosis is a new event.
- 5Discuss reproductive options and test limitations through genetics services without treating probability as a prediction of the individual outcome.
- 6Verify by enumerating four equally likely gamete pairs—A/A, A/a, a/A and a/a—which reproduces the one-two-one distribution.
02Result interpretationHandle a VUS safelyA broad panel reports a missense VUS in a gene loosely related to the patient's symptoms.+
- 1Confirm the exact laboratory classification, phenotype supplied, assay scope and whether another explanatory result was found.
- 2Do not use the VUS for predictive relative testing or alter treatment solely because the gene appears plausible.
- 3Ask the laboratory or genetics team whether segregation, phenotyping or functional evidence could meaningfully aid classification.
- 4Manage the patient by clinical phenotype and arrange re-evaluation only through a defined service process.
03Negative test reviewDefine residual risk after no variant is foundTargeted sequencing is negative despite a convincing inherited phenotype.+
- 1Check whether the correct person, indication, genes and variant classes were tested and whether sample quality was adequate.
- 2Review alternative diagnoses, phenocopies, mosaicism and mechanisms outside assay coverage.
- 3Continue phenotype-based care and family assessment where clinical risk remains.
- 4Set a genetics re-review point as knowledge, classification and available tests evolve.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- Update the pedigree when diagnoses, pregnancies or new affected relatives change the inheritance evidence.
- Retain the laboratory report, test version and consent record so later reinterpretation uses the exact assay and variant description.
- Check whether a laboratory has reclassified a material variant and ensure revised information reaches the responsible clinician and patient.
- Monitor clinically according to phenotype and established risk even when genomic testing is negative or uncertain.
- Revisit family communication and reproductive questions as relatives mature, circumstances change or new options become available.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Recurrence is conditional
A numerical risk is valid only for the stated parental genotypes, relationship, penetrance and mechanism; diagnostic uncertainty must travel with the number.
Pathogenic does not mean explanatory
A pathogenic variant can be unrelated to the presenting phenotype, particularly after broad testing, so clinical correlation remains a separate step.
One tissue can miss mosaicism
A post-zygotic variant may be enriched in affected tissue and undetectable in blood; assay detection threshold and sampled tissue shape residual risk.
Phase changes interpretation
For recessive disease, two variants must generally be on opposite homologues to affect both gene copies; parental testing may establish whether they are in trans.
Family information is relational
A result may matter to relatives, but communication must balance confidentiality, potential harm and supported disclosure through appropriate professional guidance.
07Common pitfallsFrequent interpretation and management errors.
- 01
Treating every affected family member as evidence of dominant inheritance without considering shared environment, phenocopy or ascertainment.
- 02
Giving a recurrence percentage without stating known genotypes, penetrance assumptions and whether the diagnosis is confirmed.
- 03
Calling a negative exome or genome result an exclusion of all genetic disease.
- 04
Using a VUS to change treatment, label relatives or offer predictive testing as though it were pathogenic.
- 05
Confusing variant pathogenicity with proof that the variant explains the entire phenotype.
- 06
Ordering broad testing without a consent conversation about uncertain, incidental and family-relevant outcomes.