Single-Gene Disorders: Inheritance, Mechanisms and Emergency Presentations

Single-gene disorders are caused predominantly by a pathogenic variant in one gene, although other genes and the environment modify severity. They may be inherited in autosomal dominant, autosomal recessive, X-linked, mitochondrial or less common Y-linked patterns, or arise de novo. Recognising the inheritance pattern helps clinicians anticipate complications, counsel families and choose targeted investigations without delaying emergency treatment.

Learning outcomes

The learner should be able to define a single-gene disorder, recognise Mendelian pedigrees, explain penetrance and variable expressivity, relate molecular mechanisms to phenotype, identify major emergency examples and plan safe diagnostic referral and family counselling.

1. Definition and mechanisms

A single-gene disorder results mainly from a pathogenic variant in one gene. The affected gene may encode an enzyme, receptor, ion channel, structural protein, transcription factor, haemoglobin chain or DNA-repair protein. The variant can alter the amount, location, folding, stability or function of the gene product.

Mechanism	What happens	Representative pattern
Loss-of-function	Protein activity is reduced or absent	Recessive enzyme deficiencies; some haploinsufficient dominant disorders
Gain-of-function	Protein is overactive, constitutive or has a new activity	Some receptor, signalling and skeletal dysplasias
Dominant-negative	Abnormal product interferes with the normal product	Multimeric structural proteins
Haploinsufficiency	One normal copy cannot produce enough product	Dosage-sensitive developmental disorders
Misfolding/aggregation	Protein fails to fold or accumulates in cells	Selected haemoglobin, neurodegenerative and storage diseases
Toxic gain or substrate accumulation	Abnormal metabolite injures tissues	Inborn errors of metabolism

2. Mendelian inheritance patterns

2.1 Autosomal dominant

• One pathogenic allele may cause disease; both sexes are usually affected.
• Often appears in successive generations, but a person can be the first case because of a de novo variant.
• Reduced penetrance means an apparently unaffected parent may carry the variant; variable expressivity means relatives have different severity.
• Each child of a heterozygous affected parent may have a 50% chance of inheriting the variant, but counselling must use the exact family genotype.

2.2 Autosomal recessive

• Disease generally requires pathogenic variants in both alleles; parents are often asymptomatic carriers.
• Each pregnancy of two carriers has an expected 25% affected, 50% carrier and 25% unaffected/non-carrier probability.
• Consanguinity increases the chance of shared rare alleles but is not necessary for recessive disease.
• Sibling recurrence is high while the wider pedigree may appear negative.

2.3 X-linked

Male expression may be severe because there is only one X chromosome. Female carriers may be asymptomatic or variably affected due to X-inactivation. Affected fathers do not transmit an X-linked variant to sons but transmit their X chromosome to all daughters; an affected or carrier mother has sex- and genotype-dependent risks.

2.4 Mitochondrial and de novo

Mitochondrial DNA is usually maternally inherited and may show heteroplasmy. De novo variants arise in the child and may explain a severe phenotype without a family history; parental mosaicism can still affect recurrence risk.

3. Features that modify a Mendelian pattern

• Penetrance: proportion of carriers who develop any phenotype.
• Expressivity: severity and organ pattern among affected carriers.
• Anticipation: earlier onset or greater severity with repeat expansion in some disorders.
• Imprinting: phenotype depends on parental origin.
• Mosaicism: variant burden differs among tissues.
• Phenocopy: environmental or different genetic cause produces a similar phenotype.

4. Major single-gene disorders relevant to clinical medicine

4.1 Sickle-cell disease (HBB)

An altered beta-globin chain polymerises when deoxygenated, causing red-cell sickling, haemolysis and vascular obstruction. Common emergencies include vaso-occlusive pain, acute chest syndrome, stroke, severe anaemia, splenic sequestration, priapism and sepsis.

Emergency clue	Immediate priorities
Chest pain, fever, hypoxia, new infiltrate	Oxygen if hypoxaemic, analgesia, infection assessment, respiratory support and urgent specialist input
Severe focal neurological deficit or seizure	Stroke pathway, glucose, neuroimaging and urgent transfusion/haematology guidance
Rapid splenic enlargement, pallor, shock	Resuscitation, blood count, urgent transfusion guidance
Fever in a functionally asplenic patient	Sepsis evaluation and prompt antimicrobial treatment according to protocol

Trait is usually asymptomatic but is not identical to disease. Do not infer genotype from ethnicity alone.

4.2 Cystic fibrosis (CFTR)

Defective epithelial chloride and bicarbonate transport produces thick secretions. Pulmonary infection, bronchiectasis, pancreatic insufficiency, meconium ileus, distal intestinal obstruction and salt-loss dehydration may occur. Acute respiratory distress and severe electrolyte disturbance require standard resuscitation plus cystic-fibrosis expertise.

4.3 Haemophilia A/B

X-linked factor VIII or IX deficiency causes prolonged bleeding, haemarthrosis, muscle haematoma and potentially life-threatening intracranial or airway bleeding. Treat suspected serious bleeding promptly with the patient’s replacement protocol or specialist guidance; do not wait for a laboratory result when the history is convincing. Avoid unnecessary intramuscular procedures and document inhibitor status.

4.4 Duchenne and Becker muscular dystrophy (DMD)

Variants affecting dystrophin cause progressive skeletal and respiratory muscle weakness with cardiomyopathy risk. Emergency issues include respiratory infection, cardiac decompensation, rhabdomyolysis-like episodes and anaesthetic complications. Succinylcholine and volatile anaesthesia may be hazardous in susceptible neuromuscular disease; consult anaesthesia and specialist guidance.

4.5 Phenylketonuria and other metabolic disorders

Phenylalanine hydroxylase deficiency causes toxic phenylalanine accumulation unless treated early. Other enzyme defects present with episodic vomiting, hypoglycaemia, acidosis, hyperammonaemia, seizures or encephalopathy. During acute illness, stop catabolism, provide appropriate glucose and follow the metabolic emergency protocol while obtaining critical samples when safe.

4.6 Familial hypercholesterolaemia (LDLR, APOB, PCSK9)

Reduced LDL clearance causes very high LDL cholesterol, tendon xanthomas and premature atherosclerotic disease. An acute coronary syndrome is treated conventionally, but early recognition prompts cascade family screening and long-term lipid management.

4.7 Marfan syndrome (FBN1)

Fibrillin dysfunction affects connective tissue, producing tall habitus, lens dislocation, aortic-root dilatation and skeletal features. Sudden tearing chest/back pain, pulse asymmetry or a new aortic regurgitation murmur is an aortic emergency until proven otherwise.

4.8 Thalassaemias

Reduced alpha- or beta-globin synthesis causes microcytic anaemia of variable severity. Severe disease may produce transfusion dependence, iron overload, infection risk, extramedullary haematopoiesis and cardiac complications. Do not assume every microcytic anaemia is iron deficiency; interpret ferritin, haemoglobin analysis and clinical context.

5. Genotype–phenotype relationships

• Allelic heterogeneity: different variants in one gene produce different severity, as in CFTR disorders.
• Locus heterogeneity: variants in different genes produce a similar phenotype, as in inherited hearing loss.
• Modifier genes: other loci alter age of onset or severity.
• Environment: diet, infection, toxins, altitude and treatment change expression.
• Residual function: a hypomorphic variant may preserve partial activity and produce a milder phenotype.

6. Diagnostic approach

1. Define the phenotype precisely: onset, triggers, organ systems, laboratory pattern and progression.
2. Construct a three-generation family history including sudden death, recurrent miscarriage, developmental delay, unusual anaemia, thrombosis and consanguinity.
3. Look for syndromic clues: dysmorphism, skeletal pattern, skin findings, organ malformations or recurrent metabolic crises.
4. Choose testing based on the question: familial variant, single gene, panel, exome/genome, copy-number study or biochemical assay.
5. Interpret variants with laboratory and genetics specialists. A negative result may reflect test limitations; a variant of uncertain significance is not a diagnosis.
6. Offer counselling and appropriate family testing, protecting confidentiality.

7. Emergency safety rules

8. Genetic counselling and family implications

Explain recurrence risk using the confirmed mode of inheritance and laboratory result, not appearance alone. Discuss carrier testing, reproductive options and possible findings before testing. Respect patient autonomy and avoid blaming parents for de novo variants or for inherited conditions they could not have predicted.

9. Common diagnostic pitfalls

• Assuming an absent family history excludes a genetic condition.
• Using ethnicity as a substitute for testing.
• Calling every DNA change a pathogenic mutation.
• Ordering a broad test without a plan for uncertain and incidental findings.
• Ignoring common acquired causes because a patient has a genetic diagnosis.
• Failing to document the result in a way that emergency teams can find.

References and further reading

• NCBI Bookshelf: Genes and chromosomes.
• GeneReviews: Medical genetics overview.
• NCBI Bookshelf: Sickle cell disease.
• Follow local haemoglobinopathy, bleeding, metabolic, neonatal, anaesthetic and genetic-counselling protocols.