Pathology

Benign and malignant tumours are both neoplasms, but their biological behaviour is different. Benign tumours usually remain localised, whereas malignant tumours can invade adjacent tissues and metastasise. The distinction is based on integrated microscopic, macroscopic, molecular and clinical evidence—not on size, pain or growth speed alone. A benign lesion can still threaten life by compressing vital structures, bleeding, obstructing flow or secreting hormones. Core comparison Feature Benign Malignant Growth Often slow and expansile Variable; often progressive and infiltrative Differentiation Usually resembles tissue of origin Ranges from well differentiated to anaplastic Local invasion Absent Characteristic capability Metastasis Absent by definition Possible; major cause of morbidity and mortality Recurrence Usually linked to incomplete removal More likely through local persistence or distant disease Learning outcomes The learner should be able to compare benign and malignant neoplasms, describe differences in growth, differentiation, margins, mitoses, necrosis and spread, recognise important exceptions, understand grade and stage, and triage a patient with a potentially dangerous mass. 1. Growth pattern Benign tumours Benign tumours often grow by expansion, pushing adjacent tissue aside. A fibrous capsule or pseudocapsule may form from compressed stroma. The lesion may be mobile relative to surrounding tissue, but this is not universal and does not prove benignity. Malignant tumours Malignant tumours infiltrate along planes, vessels, nerves and tissue compartments. They may be fixed, irregular or ulcerated, but some are deceptively circumscribed. Microscopic extension can exceed the palpable mass, making imaging and margin assessment important. Why growth rate is not enough Some benign lesions grow quickly under hormonal stimulation, while some cancers are indolent for years. A rapidly enlarging mass is concerning, but a stable or painless lump still requires appropriate assessment when persistent or changing. 2. Differentiation and anaplasia Microscopic feature Well-differentiated/benign tendency Poorly differentiated/malignant tendency Cell size and shape Uniform and similar to parent tissue Pleomorphism and marked variation Nuclei Regular, proportionate and less hyperchromatic Enlarged, hyperchromatic, irregular nuclei Polarity Preserved organisation Loss of polarity and architecture Mitoses Few, normal forms Increased and/or atypical mitoses Specialisation Retained function may be evident May lose function or produce abnormal products Necrosis Uncommon unless large or torsed More likely when growth outstrips blood supply Anaplasia means lack of differentiation. It strongly supports malignancy but must be interpreted with the organ-specific diagnostic criteria and the possibility of severe reactive atypia. 3. Margins, capsule and local invasion Benign margin: commonly smooth, circumscribed or encapsulated; adjacent tissue may be compressed rather than destroyed. Malignant margin: irregular, infiltrative, satellite nests or tongues of cells in surrounding tissue; perineural and vascular invasion may be present. Important exception: a pseudocapsule can surround a malignant lesion, and a benign lesion may be poorly circumscribed. Histology and imaging, not palpation alone, determine behaviour. Invasion: malignant cells cross basement membrane, degrade extracellular matrix and enter adjacent organs, lymphatics or vessels. 4. Metastasis Metastasis is a discontinuous tumour deposit at a distant site. It involves local invasion, intravasation, survival in circulation, extravasation and colonisation. Lymphatic spread is common for carcinomas; blood-borne spread is common for sarcomas and many carcinomas; body-cavity seeding occurs in selected tumours. Metastasis is not required for a tumour to be malignant—an invasive primary cancer may be malignant before distant spread is detected. Conversely, absence of metastasis on initial imaging does not prove cure. 5. Clinical differences Clinical feature More typical of benign disease More concerning for malignancy Mass Long-standing, mobile, well circumscribed Progressive enlargement, fixation, irregularity or satellite nodules Surface Intact and stable Ulceration, bleeding, crusting or non-healing wound Symptoms Pressure or hormone effect without invasion Persistent pain, nerve deficit, obstruction or systemic features Nodes Reactive nodes may be tender and mobile Hard, fixed, progressive or anatomically linked nodes Systemic effects Usually absent unless functional Weight loss, fever, anaemia, thrombosis or paraneoplastic syndrome These are clues, not diagnostic rules. Infections, inflammatory lesions and benign masses can mimic cancer, while early malignancy can be subtle. 6. Grade, stage and prognosis Grade Grade describes microscopic differentiation, mitotic activity, necrosis and other organ-specific features. High grade often correlates with aggressive behaviour, but grading systems differ between cancers. Stage Stage describes how far the tumour has spread—primary tumour extent, regional nodes and distant metastasis. It often guides treatment and prognosis more directly than grade. A small high-grade cancer and a large low-grade cancer are not equivalent. Other prognostic factors Margin status, lymphovascular/perineural invasion and tumour subtype. Molecular alterations, receptor status and response to treatment. Patient comorbidities, functional status and access to follow-up. 7. Exceptions students must remember Benign does not mean harmless: a meningioma can compress the brain; a pituitary adenoma can impair vision or hormones; a uterine leiomyoma can cause severe bleeding. Some malignant tumours do not form a lump: leukaemia spreads through marrow and blood; lymphoma may be diffuse; carcinomatosis coats serosal surfaces. Some tumours with “-oma” are malignant: melanoma, lymphoma, mesothelioma and seminoma. Size is not behaviour: a small invasive carcinoma can be dangerous, while a very large benign tumour may remain localised. Pain is not a reliable discriminator: benign lesions can hurt through pressure, torsion or haemorrhage; early cancer may be painless. 8. Diagnostic pathway History: duration, rate of growth, pain, bleeding, systemic symptoms, exposures, family history and prior cancer. Examination: site, size, depth, mobility, skin changes, nodes, neurovascular status and signs of obstruction or infection. Imaging: ultrasound for superficial lesions, radiography/CT/MRI for anatomy and staging, selected nuclear imaging where indicated. Tissue: core, incisional, excisional or cytological sampling chosen to preserve future treatment options and avoid contaminating compartments. Pathology: morphology, immunohistochemistry, flow cytometry or molecular testing as appropriate. Multidisciplinary review: integrate diagnosis, grade, stage, patient fitness and treatment goals. 9. Emergency presentations Neurological emergency Spinal cord compression: new back pain, limb weakness, sensory level or bladder/bowel dysfunction. Raised intracranial pressure, seizures or acute neurological deficit. Urgent imaging and specialist escalation. Airway/vascular emergency Stridor, airway obstruction, superior vena cava syndrome or major haemorrhage. Pericardial tamponade or malignant pleural effusion causing respiratory compromise. Stabilise ABCDE and obtain urgent imaging/drainage support. Metabolic/haematological emergency Hypercalcaemia, tumour lysis syndrome, neutropenic sepsis or hyperleukocytosis. Severe anaemia, thrombocytopenic

A tumour is an ecosystem, not just a lump of malignant cells. Its neoplastic parenchyma grows within a supporting stroma of vessels, fibroblasts, extracellular matrix and immune cells. The interaction between these components determines growth, necrosis, invasion, treatment response and clinical presentation. Understanding the components and characteristics of tumours helps the clinician interpret a mass, communicate with pathology and recognise urgent complications. Core structure Parenchyma: the neoplastic cells; determines lineage, tumour name and many molecular behaviours. Stroma: blood vessels, lymphatics, fibroblasts, matrix and inflammatory cells that support or restrain growth. Microenvironment: dynamic communication among tumour, immune, vascular and stromal cells. Characteristics: clonal growth, altered differentiation, autonomy, angiogenesis, immune evasion, metabolic adaptation, invasion and—when malignant—metastasis. Learning outcomes The learner should be able to identify the parenchymal and stromal components of a tumour, explain the tumour microenvironment, describe the major biological hallmarks of cancer, correlate microscopic and macroscopic features with clinical behaviour, and recognise features that require urgent evaluation. 1. Parenchyma: the neoplastic cell population The parenchyma consists of transformed cells derived from a founding clone. It determines the tumour’s lineage and is assessed by morphology, immunohistochemistry, flow cytometry and molecular studies. Tumour cells may be uniform or heterogeneous because additional alterations create subclones over time. 1.1 Differentiation Well-differentiated cells resemble their tissue of origin and may retain specialised functions such as mucus, keratin, hormones or melanin production. Poorly differentiated or anaplastic cells show little resemblance to normal tissue and often behave more aggressively, although grade and prognosis are organ-specific. 1.2 Clonality and heterogeneity Many tumours begin from one altered cell, but the clone evolves. Regions of the same tumour may differ in mutations, grade, oxygenation and immune infiltration. Sampling error can therefore miss a clinically important subclone; pathology integrates the specimen with imaging and clinical findings. 2. Stroma: the tumour’s supporting framework Stromal component Role Clinical effect Blood vessels/endothelial cells Deliver oxygen and nutrients; remove waste Support growth, bleeding and haematogenous spread Lymphatics Drain interstitial fluid and provide a route to nodes Enable regional metastasis Fibroblasts Produce extracellular matrix and growth factors Desmoplasia, firmness and altered drug penetration Extracellular matrix Provides structure and biochemical signals Controls invasion, migration and tissue stiffness Macrophages and other immune cells Can attack tumour or promote growth and repair-like responses Immune surveillance, inflammation and immune escape Pericytes and smooth muscle cells Stabilise vessels and regulate permeability Abnormal vessels may leak, thrombose or support invasion 3. Tumour microenvironment Tumour cells release cytokines, chemokines, growth factors and extracellular vesicles that recruit and reprogramme stromal cells. Fibroblasts may become cancer-associated fibroblasts; macrophages may acquire tumour-supporting functions; endothelial cells form abnormal vessels. Low oxygen selects cells able to survive hypoxia and stimulates angiogenic signals. The microenvironment can also suppress T-cell activity and reduce drug delivery. 4. Major characteristics and hallmarks 4.1 Sustaining proliferative signalling Normal cells require regulated growth signals. Tumour cells may produce their own signals, amplify receptors or activate downstream pathways such as RAS–MAPK and PI3K–AKT. The result is persistent division despite absent normal stimuli. 4.2 Evading growth suppressors Normal tissues use checkpoints and tumour-suppressor proteins to limit proliferation. Loss of pathways involving RB, p53, TGF-beta or contact inhibition allows cells with damage or crowding to continue dividing. 4.3 Resisting cell death Apoptosis removes dangerous or damaged cells. Tumours may lose pro-apoptotic signals, increase anti-apoptotic proteins, avoid immune killing or adapt to hypoxia. Necrosis and autophagy can also influence survival and inflammation. 4.4 Enabling replicative immortality Most somatic cells have limited division because telomeres shorten. Many cancers reactivate telomerase or alternative lengthening mechanisms, allowing repeated replication. 4.5 Inducing or co-opting angiogenesis A growing mass requires new or recruited vessels. Hypoxia-inducible factors and vascular endothelial growth factor stimulate angiogenesis. Tumour vessels are irregular, fragile and leaky, causing haemorrhage, oedema, hypoxia and variable drug delivery. 4.6 Activating invasion and metastasis Malignant cells reduce adhesion, degrade basement membranes, migrate through stroma, enter vessels, survive circulation and colonise distant organs. Organ tropism depends on blood flow, adhesion molecules, chemokines and the “seed-and-soil” relationship between tumour cells and target tissue. 4.7 Avoiding immune destruction Tumours can reduce antigen presentation, express inhibitory ligands, recruit regulatory immune cells and create an immunosuppressive microenvironment. Immune surveillance still eliminates many abnormal cells, but successful clones evolve ways to escape. 4.8 Deregulating cellular metabolism Many tumours favour aerobic glycolysis and increase glucose uptake even when oxygen is available (the Warburg effect). Metabolic rewiring supports rapid biosynthesis and survival in a changing microenvironment, but it also creates imaging and therapeutic vulnerabilities. 4.9 Maintaining genome instability Defects in DNA repair, chromosome segregation or cell-cycle checkpoints increase mutation and structural variation. Genome instability accelerates evolution of resistant subclones and may create targets for therapy. 4.10 Tumour-promoting inflammation Chronic inflammatory cells and mediators can stimulate proliferation, angiogenesis, tissue remodelling and immune suppression. Infection, smoking, reflux, inflammatory bowel disease and other chronic injuries may create a cancer-promoting environment in susceptible tissues. 5. Macroscopic characteristics Gross feature Possible meaning Clinical correlation Well circumscribed/encapsulated Expansile growth and compression Often benign, but not proof of benignity Infiltrative irregular edge Extension through normal tissue Suggests malignancy and incomplete excision risk Firm, white, fibrous cut surface Desmoplastic stroma Hard palpable lesion; common in some carcinomas Soft, friable, haemorrhagic Rapid growth, fragile vessels or necrosis Bleeding, anaemia and urgent deterioration Central necrosis/cavitation Growth outstrips blood supply Fever, pain, foul discharge or fistula may occur Ulceration Tumour breaks through surface epithelium Bleeding, infection, pain and difficulty healing Satellite nodules or nodes Local lymphatic or discontinuous spread Staging and biopsy of appropriate site 6. Microscopic characteristics Pleomorphism and variation in cell and nuclear size. Hyperchromatic enlarged nuclei and a high nuclear-to-cytoplasmic ratio. Loss of polarity and architectural disorganisation. Abnormal mitotic figures and increased proliferation index. Necrosis, apoptotic debris and inflammatory infiltrates. Invasion through basement membrane, nerves, vessels or adjacent organs. Specialised products such as keratin pearls, mucin, hormones or pigment may identify differentiation. These findings are interpreted within organ-specific criteria. Atypia can occur in reactive conditions, and not every high mitotic rate means malignancy. 7. Tumour growth patterns Expansile growth

Neoplasia is abnormal, clonal cell proliferation whose growth is excessive and uncoordinated with normal tissues and continues even after the initiating stimulus has stopped. The resulting mass is a neoplasm, commonly called a tumour. Tumours may be benign or malignant; malignant neoplasms are cancers that invade tissues and may metastasise. Essential terminology Term Meaning Key point Neoplasia The process of abnormal autonomous proliferation Growth persists after the original trigger is removed Neoplasm An abnormal mass produced by neoplasia May be benign or malignant Tumour Common clinical term for a neoplasm or mass Not every mass is a neoplasm Cancer Malignant neoplasm Can invade and metastasise; blood cancers may not form a solid mass Benign Non-malignant growth Does not invade or metastasise, but can still be dangerous by compression, obstruction or hormone production Learning outcomes By the end of this page, the learner should be able to define neoplasia, distinguish neoplasm from hyperplasia and dysplasia, identify parenchymal and stromal components, compare benign and malignant tumours, explain differentiation, anaplasia, invasion, metastasis, grade and stage, and recognise urgent presentations caused by tumours. 1. What is neoplasia? Neoplasia literally means “new growth.” A neoplastic proliferation results from acquired or inherited genetic and epigenetic alterations that give a cell and its descendants a growth advantage. The abnormal clone may remain localised and benign or acquire the capabilities of invasion, immune evasion, angiogenesis and metastasis. Neoplasia is different from a normal physiological response. A wound-healing cell population usually stops proliferating when repair is complete. A neoplastic clone becomes relatively autonomous and may continue to expand despite inhibitory signals, nutrient limitations or contact with neighbouring cells. 2. Neoplasia compared with other growth disturbances Process Definition Reversibility/behaviour Hypertrophy Increase in cell size Usually adaptive and reversible when the stimulus resolves Hyperplasia Increase in cell number in response to a stimulus Usually controlled and may regress; some settings increase cancer risk Metaplasia Reversible replacement of one mature cell type by another better suited to stress Protective initially but can predispose to dysplasia if injury persists Dysplasia Disordered growth and maturation with cytological atypia May regress or progress; often a premalignant change, not synonymous with cancer Neoplasia Clonal autonomous proliferation Persists after the initiating stimulus; benign or malignant 3. Components of a tumour 3.1 Parenchyma The parenchyma consists of the neoplastic cells. It determines the tumour’s name, differentiation, hormone production, molecular profile and many biological behaviours. For example, the neoplastic epithelial cells of a carcinoma differ from the malignant lymphoid cells of a lymphoma. 3.2 Stroma The stroma is the supporting tissue: extracellular matrix, fibroblasts, blood vessels, lymphatics and immune cells. A tumour cannot grow beyond a small size without inducing or co-opting a blood supply. Stromal fibrosis can make a carcinoma firm; inflammatory and immune cells may either restrain or support tumour growth. 3.3 Tumour microenvironment Cancer cells interact with fibroblasts, endothelial cells, macrophages, lymphocytes and extracellular matrix. This microenvironment influences invasion, drug penetration, immune escape and metastasis. Therefore, tumour behaviour is not determined by malignant cells alone. 4. Classification by tissue of origin Origin Benign example/name Malignant example/name Epithelial Adenoma, papilloma, cystadenoma Carcinoma, adenocarcinoma, squamous-cell carcinoma Mesenchymal/connective Lipoma, fibroma, chondroma, leiomyoma Sarcoma, liposarcoma, osteosarcoma, leiomyosarcoma Haematolymphoid Some benign lymphoid proliferations Leukaemia, lymphoma, plasma-cell myeloma Melanocytic Melanocytic naevus Melanoma Germ-cell Some mature teratomas Embryonal carcinoma, yolk-sac tumour and other malignant germ-cell tumours Embryonal Rare benign counterparts Blastomas such as nephroblastoma or neuroblastoma Naming exceptions exist. Some tumours ending in “-oma,” such as melanoma, lymphoma and mesothelioma, are malignant; pathology and clinical context determine behaviour. 5. Characteristics of benign neoplasms Often well circumscribed, slow growing and expansile rather than infiltrative. Cells may resemble the tissue of origin and retain specialised function. Mitotic figures are usually few and typical. They do not invade nearby tissue or metastasise, although local recurrence can occur if incompletely removed. They can cause severe disease through compression of the brainstem, airway, spinal cord, bile duct or blood vessels; by obstruction; by haemorrhage; or by hormone production. 6. Characteristics of malignant neoplasms Variable differentiation, often with atypia and anaplasia. Rapid or irregular growth, although some malignancies are indolent. Infiltration of adjacent tissue and destruction of normal architecture. Ability to invade lymphatics, blood vessels or body cavities and metastasise. Necrosis, haemorrhage, ulceration and a desmoplastic or inflammatory stroma may occur. Recurrence after incomplete local treatment is more likely. Growth rate alone does not distinguish benign from malignant. Some benign lesions grow quickly, while some malignant tumours grow slowly. Invasion and metastatic potential are central definitions of malignancy. 7. Differentiation and anaplasia 7.1 Differentiation Differentiation is the extent to which neoplastic cells resemble their normal counterparts structurally and functionally. A well-differentiated adenocarcinoma may form glands and secrete mucin; a poorly differentiated tumour may show little evidence of its origin. 7.2 Anaplasia Anaplasia means lack of differentiation and is a feature of aggressive malignancy. Histological clues include pleomorphism, enlarged hyperchromatic nuclei, a high nuclear-to-cytoplasmic ratio, abnormal mitoses, tumour giant cells, loss of polarity and necrosis. These findings support but do not replace integrated pathological diagnosis. 8. In situ disease, invasion and metastasis 8.1 Carcinoma in situ Carcinoma in situ displays cytological malignancy across the full thickness of an epithelium but has not breached the basement membrane. It has no access to lymphatic or blood vessels through an invasive front, although it may progress to invasive cancer if untreated. Not all in-situ lesions behave identically; management depends on organ and pathology. 8.2 Local invasion Invasion requires loss of adhesion, degradation of extracellular matrix, migration through stroma and survival in a new microenvironment. Malignant cells can penetrate basement membranes, nerves, vessels and adjacent organs. Perineural invasion can cause pain or neurological deficit; vascular invasion increases metastatic risk. 8.3 Metastasis Metastasis is the formation of a discontinuous tumour deposit at a distant site. The sequence includes local invasion, intravasation, survival in the circulation, extravasation and colonisation. Routes include lymphatic spread, haematogenous spread and seeding of body cavities. Metastases often retain the histological identity of the primary tumour but may evolve new molecular

Multifactorial (complex or polygenic) disorders arise from the combined effects of many genetic variants, environmental exposures, behaviours, developmental events and social conditions. Unlike a classic single-gene disorder, no single variant is usually sufficient or necessary. The result is a spectrum of susceptibility: a person may inherit increased risk without developing disease, while another person with little apparent family history may become ill after a powerful environmental exposure. At a glance Polygenic: many variants each make a small contribution to a trait or disease. Multifactorial: genetic susceptibility interacts with environmental, developmental, behavioural and social factors. Familial clustering is not proof of a single gene: relatives share genes, diet, exposures, culture and access to care. Risk is probabilistic: family history raises or lowers risk but does not predict an individual’s outcome with certainty. Clinical approach: identify modifiable risks, screen according to evidence-based guidelines and treat acute illness normally while considering inherited susceptibility. Learning outcomes By the end of this page, the learner should be able to distinguish multifactorial, polygenic and monogenic disease; explain the threshold model and gene–environment interaction; interpret family history and heritability correctly; recognise common examples and developmental conditions; discuss prevention and screening; and apply a safe emergency approach. 1. Definitions and related terms Term Meaning Example or implication Polygenic A trait is influenced by variants at many loci Blood pressure, type 2 diabetes susceptibility Multifactorial/complex Multiple genes interact with environment and development Coronary disease, asthma, cleft lip/palate Oligogenic A small number of genes have major combined effects Some congenital and endocrine disorders Familial aggregation A condition occurs more often among relatives May reflect genes, shared environment or both Heritability Proportion of variation in a population attributable to genetic differences in that setting Not the percentage of an individual’s disease caused by genes Risk allele An allele statistically associated with increased susceptibility Usually has a small effect and is not diagnostic alone Protective factor Genetic, behavioural or environmental factor lowering risk Exercise, vaccination or a protective allele 2. How complex disease develops 2.1 Many small genetic effects Most susceptibility variants are common and individually weak. Each may alter gene expression, protein activity, immune signalling, lipid handling, insulin response or vascular function by a small amount. Disease appears when the total liability crosses a biologic or clinical threshold. 2.2 Gene–environment interaction The same exposure can have different effects in people with different genotypes, and the same genetic susceptibility can produce different outcomes in different environments. Examples include air pollution and asthma, diet and type 2 diabetes, smoking and coronary disease, or folate status and neural-tube defects. An association does not prove that one factor caused the individual’s disease. 2.3 Developmental timing Risk factors may act before conception, during embryonic development, in childhood or in adult life. Maternal diabetes, folate status, infection, medicines, alcohol, nutrition and placental function can influence congenital outcomes. Later exposures may determine whether a predisposition becomes clinically apparent. 2.4 Epigenetic and microbiome effects DNA methylation, histone modification, chromatin structure, non-coding RNA, immune history and the microbiome can alter gene expression without changing the DNA sequence. These effects are context-dependent and should not be described as proof that a parent’s behaviour “caused” a child’s condition. 3. The multifactorial threshold model Many congenital conditions follow a threshold model. Genetic and environmental liabilities add together on a continuous scale. Disease occurs when total liability exceeds a threshold. A family may have several affected members even though no single Mendelian variant is found. More severe disease may imply a higher familial liability. If the condition is more common in one sex, an affected person in the less commonly affected sex may represent a higher familial load (the Carter effect). Recurrence risk is usually empirical and depends on the specific condition, number of affected relatives and severity. The model is useful for counselling but does not give an exact prediction for every family. 4. Heritability: what it means and what it does not mean Heritability is a population statistic: it estimates how much of the variation in a trait, in a particular population and environment, is associated with genetic differences. It does not mean that a fixed percentage of an individual’s disease is genetic, that the trait is unchangeable or that environmental interventions are ineffective. Heritability can change when environments change. 5. Common multifactorial disorders 5.1 Cardiometabolic disease Hypertension: kidney salt handling, vascular tone, neurohormonal regulation, obesity, dietary sodium, alcohol, sleep and stress interact. Type 2 diabetes: beta-cell reserve, insulin sensitivity, adiposity, diet, activity, sleep, medications and social conditions contribute. A family history increases risk but is not destiny. Coronary artery disease: lipid metabolism, blood pressure, inflammation, smoking, diabetes, diet and age interact. A monogenic mimic such as familial hypercholesterolaemia should be considered when LDL is very high or disease is unusually early. Obesity: appetite regulation, energy expenditure, sleep, food environment, medicines, stress and socioeconomic factors contribute; avoid blaming individuals for a biologically and socially complex condition. 5.2 Respiratory and allergic disease Asthma susceptibility reflects airway biology, atopy, viral infection, allergens, smoke, occupational exposures, pollution, obesity and access to controller treatment. An acute severe attack is managed according to severity, not by genetic testing. 5.3 Autoimmune and inflammatory disease HLA and other immune variants interact with infections, hormones, smoking, microbiome and tissue injury in conditions such as type 1 diabetes, rheumatoid arthritis, systemic lupus and inflammatory bowel disease. Risk alleles are neither necessary nor sufficient. 5.4 Neuropsychiatric disease Schizophrenia, epilepsy, depression, autism and many neurodevelopmental conditions involve many variants, rare variants, development and environment. Genetic susceptibility should never be used to dismiss psychosocial care or to make deterministic predictions. 6. Multifactorial congenital conditions Condition Contributing factors Prevention/clinical focus Neural-tube defects Polygenic susceptibility, folate status, diabetes, some medicines, fever and other factors CDC states that 400 micrograms of folic acid daily before and during early pregnancy helps prevent neural-tube defects; high-risk regimens require clinician guidance Cleft lip with or without palate Multiple genes, maternal exposures, nutrition and developmental timing Detailed anomaly assessment, feeding/airway support and multidisciplinary care Congenital heart disease Multiple genetic and

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. At a glance Monogenic does not mean simple: one gene can affect several organs (pleiotropy), and one gene may have many different variants (allelic heterogeneity). Mechanisms: loss-of-function, gain-of-function, dominant-negative effect, haploinsufficiency, abnormal protein folding, toxic accumulation or defective DNA repair. Inheritance: autosomal dominant/recessive, X-linked, mitochondrial, Y-linked or de novo. Emergency principle: stabilise airway, breathing, circulation, glucose, seizures, pain or sepsis first; genetic confirmation follows. 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

Genetic disorders are conditions in which a change in DNA, chromosome structure/number, or heritable gene regulation contributes to disease. They are classified by molecular mechanism and inheritance because those categories predict recurrence risk, clinical pattern, testing strategy and emergency priorities. A disorder may fit more than one category—for example, a chromosomal deletion can cause a contiguous-gene syndrome with multifactorial modifiers. Classification at a glance Single-gene (monogenic): one gene has a major causal variant; may be autosomal dominant, autosomal recessive, X-linked, Y-linked or mitochondrial. Chromosomal: a whole chromosome or large segment is gained, lost or rearranged. Multifactorial/polygenic: many variants interact with each other and the environment. Mitochondrial and metabolic: variants disrupt energy production or biochemical pathways; inheritance and tissue distribution may be distinctive. Epigenetic/imprinting: gene activity changes without altering DNA sequence, sometimes in a parent-of-origin pattern. Somatic/mosaic: variants arise after conception in a subset of cells and may cause cancer or segmental disease. Learning outcomes The learner should be able to classify genetic disorders by mechanism and inheritance, compare their clinical clues, select appropriate investigations, recognise when a genetic condition is an emergency and explain recurrence risk without deterministic or stigmatising language. 1. Why classification matters Classification links mechanism to action. A newborn with multiple congenital anomalies may need a chromosome study or microarray rather than a single-gene test. A child with recurrent hypoglycaemia and acidosis may need metabolic investigation. A family with sudden cardiac death may require targeted gene evaluation and urgent relatives’ screening. Correct classification also prevents false reassurance from a negative test that did not examine the relevant type of variant. 2. Single-gene (monogenic or Mendelian) disorders In monogenic disease, a pathogenic variant in one gene has a major effect, although severity can be modified by other genes and environment. The gene may encode an enzyme, structural protein, receptor, channel, transcription factor or DNA-repair component. 2.1 Autosomal dominant One pathogenic allele can cause disease; males and females are usually affected. Often appears in successive generations, but reduced penetrance, new variants and variable expressivity can obscure the pedigree. Examples include Marfan syndrome, familial hypercholesterolaemia, achondroplasia and many channelopathies. Each child of a heterozygous affected parent may have a 50% chance of inheriting the variant; the chance of clinical disease depends on penetrance. 2.2 Autosomal recessive Pathogenic variants in both alleles are usually required. Parents may be unaffected carriers. Consanguinity increases the likelihood that both parents share a rare ancestral allele, but recessive disease occurs in all populations. Examples include cystic fibrosis, sickle-cell disease, phenylketonuria and many inborn errors of metabolism. Two carrier parents have, for each pregnancy, a 25% chance of an affected child, 50% chance of a carrier and 25% chance of inheriting neither familial variant; actual counselling should consider the confirmed genotype. 2.3 X-linked Males have one X chromosome and may express a pathogenic variant without a second copy. Female expression varies with X-inactivation, variant type and tissue distribution. Examples include Duchenne muscular dystrophy, haemophilia and some intellectual-disability syndromes. Avoid assuming every male child is affected or every female carrier is asymptomatic. 2.4 Y-linked and pseudoautosomal Y-linked variants pass from father to son and affect Y-linked functions. Pseudoautosomal genes are present on both X and Y chromosomes and may show different inheritance behaviour. 3. Chromosomal disorders Chromosomal disorders involve extra, missing or rearranged chromosome material. They may be numerical or structural and can be constitutional (present from conception) or acquired in tumour cells. Category Mechanism Examples/clinical pattern Aneuploidy Gain or loss of an individual chromosome Trisomy 21, 18, 13; 45,X; 47,XXY Polyploidy Extra complete chromosome set Triploidy; usually severe multisystem disease or pregnancy loss Deletion Loss of a chromosome segment Contiguous-gene syndromes and developmental delay Duplication Extra copy of a segment Gene-dosage disorders Translocation Exchange between chromosomes Balanced carrier, unbalanced offspring or cancer fusion Inversion/ring/isochromosome Rearrangement within a chromosome Variable congenital, developmental or reproductive effects Mosaic chromosome abnormality Post-zygotic error in a cell line Severity depends on tissues and abnormal-cell burden Chromosomal syndromes often involve multiple organ systems because many genes are affected. Use karyotype, FISH, microarray or sequencing according to the suspected abnormality. 4. Multifactorial and polygenic disorders Common diseases such as hypertension, type 2 diabetes, asthma, coronary disease, neural-tube defects and many autoimmune conditions arise from the combined effect of many variants and environmental exposures. Risk is distributed continuously rather than following a simple dominant/recessive ratio. Family history increases risk but does not prove a single-gene cause. Nutrition, infection, smoking, stress, physical activity, toxins and socioeconomic factors modify expression. Recurrence risk is empirical and depends on the number of affected relatives, severity and population prevalence. Polygenic risk scores are population-dependent and should not replace clinical assessment or equitable care. A patient with a strong family history of early myocardial infarction, for example, may have familial hypercholesterolaemia (monogenic), polygenic risk, shared diet and environment, or a combination. 5. Mitochondrial and metabolic disorders 5.1 Mitochondrial DNA disorders Mitochondria produce ATP and contain their own DNA. Mitochondrial variants are usually maternally inherited, and heteroplasmy creates variable tissue involvement. High-energy organs—brain, skeletal muscle, heart, retina and endocrine organs—are commonly affected. A patient may present with seizures, lactic acidosis, cardiomyopathy, stroke-like episodes, deafness or ophthalmoplegia. 5.2 Inborn errors of metabolism Many metabolic disorders are autosomal recessive and cause accumulation of toxic substrates or deficiency of essential products. Examples include urea-cycle disorders, organic acidemias, fatty-acid oxidation disorders, glycogen-storage disease and lysosomal storage disease. Red flags in a sick infant Unexplained vomiting, lethargy, seizures or coma after a symptom-free interval. Hypoglycaemia, high-anion-gap acidosis, hyperammonaemia or unexplained liver failure. Recurrent decompensation during fasting or infection. Unusual odour, consanguinity or previous sibling death. Immediate principles Stabilise airway, breathing, circulation and glucose. Stop fasting and obtain critical samples before treatment when safe. Consult metabolic specialists and follow local emergency protocols. Do not delay resuscitation while awaiting genetic confirmation. 6. Epigenetic and imprinting disorders Epigenetics changes gene expression without changing the DNA letters. DNA methylation, histone modifications, chromatin remodelling and non-coding RNAs can silence or activate genes. Environmental and developmental signals influence these marks. Genomic

A mutation is a change in genetic material. Modern clinical practice often uses the more neutral term genetic variant, because a change may be benign, protective or disease-causing. Mutations arise spontaneously during DNA replication or after environmental damage, and they may occur in germ cells, somatic tissues or both. Their consequences depend on location, size, gene function, zygosity, timing and the body’s DNA-repair systems. At a glance Germline variant: present in an egg or sperm (or their precursors) and potentially transmitted to descendants. Somatic variant: acquired in body cells; usually not inherited by children but may drive cancer or mosaic disease. Point variant: a single-base change; may be synonymous, missense, nonsense or splice-altering. Indel: insertion or deletion; a frameshift can change every downstream codon. Loss-of-function, gain-of-function and dominant-negative describe functional consequences, not simply the size of a DNA change. A laboratory finding is not automatically pathogenic; classification requires population, computational, functional, segregation and clinical evidence. Learning outcomes The learner should be able to define mutation and variant, classify sequence and structural changes, explain spontaneous and induced causes, distinguish germline from somatic mosaicism, outline DNA-repair pathways, describe how mutations alter protein and cell behaviour, and interpret basic clinical and laboratory implications. 1. Terminology: mutation, variant and polymorphism Mutation traditionally means a permanent change in DNA sequence. It remains useful in terms such as “mutation rate,” “driver mutation” and “mutation-induced disease,” but it can incorrectly imply harm. Variant is preferred when significance is unknown. A polymorphism is a variant common enough in a population to be considered a normal population difference; a common allele can still influence disease susceptibility or drug response. Variants may be benign, likely benign, of uncertain significance, likely pathogenic or pathogenic. These categories describe current evidence, not a patient’s worth or destiny. 2. How mutations arise 2.1 Replication and proofreading errors DNA polymerases copy billions of bases during cell division. A mispaired base may escape proofreading and become permanent after another replication cycle. Short repeated sequences are especially prone to polymerase slippage, producing small insertions, deletions or repeat expansions. 2.2 Spontaneous chemical changes Deamination: cytosine can deaminate to uracil; methylated cytosine can deaminate to thymine, creating common transition mutations. Depurination: loss of a purine leaves an abasic site that can be misread. Oxidative damage: reactive oxygen species modify bases such as guanine and create strand breaks. Replication stress: stalled forks can collapse or be repaired inaccurately. 2.3 Environmental and occupational mutagens Agent Typical DNA damage Clinical context Ultraviolet radiation Thymine dimers and nucleotide distortion Skin cancer risk; photosensitive disorders Ionising radiation Single- and double-strand breaks Radiotherapy, occupational or accidental exposure Tobacco smoke components DNA adducts, oxidative injury and strand damage Multiple smoking-related cancers Industrial chemicals and solvents Adducts, cross-links or oxidative damage Risk depends on agent, dose and duration Some infectious agents Chronic inflammation, insertional effects or viral oncogenes HPV, hepatitis viruses and other cancer-associated infections Risk depends on dose, route, timing, metabolism, tissue susceptibility and protective measures. A suspected exposure does not prove that a particular mutation caused disease. 3. Types of genetic change 3.1 Single-nucleotide variants A transition replaces a purine with a purine or a pyrimidine with a pyrimidine; a transversion exchanges the two classes. The same nucleotide change can be harmless in one location and pathogenic in another. 3.2 Insertions and deletions An insertion or deletion of three bases (or a multiple of three) may add or remove amino acids without shifting the reading frame. A non-multiple-of-three change causes a frameshift and often introduces an early stop codon. Small indels can also alter promoters, splice sites or untranslated regions. 3.3 Repeat expansions Trinucleotide and other repeat sequences can expand during gamete formation or early development. Some disorders show anticipation—earlier onset or greater severity in successive generations—although the pattern depends on the specific repeat and gene. 3.4 Copy-number and structural variants Deletions and duplications change gene dosage. Inversions can disrupt a gene or impair pairing during meiosis. Translocations can create gene fusions or regulatory changes. Large insertions, mobile-element insertions and complex rearrangements may be missed by routine sequencing. 4. Where the mutation occurs 4.1 Germline A germline variant is present in the egg or sperm, or arises very early after fertilisation. It can be present in most cells and passed to children. A parent may be unaffected because of reduced penetrance, sex-dependent expression or a recessive genotype. 4.2 Somatic Somatic variants arise after conception in body cells. They can create a clone of abnormal cells, contribute to cancer or produce segmental/mosaic disease. They are generally not transmitted to offspring, although a gonadal mosaic variant can be passed on despite being absent from blood. 4.3 Mosaicism Mosaicism means two or more genetically different cell lines in one individual. The phenotype depends on the timing of the event, affected tissues, variant burden and selective advantage of the abnormal cells. A negative blood test does not exclude a tissue-limited mosaic condition. 5. Functional consequences Consequence Mechanism Example pattern Loss-of-function Reduced or absent protein activity One normal allele may be enough, or disease appears when both are affected Haploinsufficiency One working copy cannot produce enough product Often dominant dosage disorders Gain-of-function Increased, constitutive or new activity Some receptor or signalling disorders Dominant-negative Abnormal protein interferes with the normal product Structural multimeric proteins Neomorphic New biochemical function Selected enzyme or signalling changes Regulatory change Altered timing, tissue or amount of expression Enhancer or promoter variant 5.1 Protein-level effects Synonymous: amino acid unchanged, but codon usage, splicing or RNA stability may be altered. Missense: amino acid substitution; effect depends on conservation, structure and biochemical properties. Nonsense: premature stop; transcript may be degraded or a truncated protein produced. Splice-altering: exon skipping, intron retention or cryptic splice-site use. Start-loss/stop-loss: translation starts incorrectly or continues beyond the normal stop. 6. DNA damage and repair 6.1 Proofreading and mismatch repair Replicative polymerases correct many errors immediately. Mismatch-repair proteins detect remaining mispaired bases and correct the newly synthesised strand. Inherited mismatch-repair defects increase the risk of certain cancers. 6.2 Base-excision repair DNA glycosylases remove damaged

A chromosome is an organised DNA–protein structure that packages and transmits genetic information. Human chromosomes carry thousands of genes and regulatory sequences. Their number, structure and behaviour during mitosis and meiosis explain normal growth, congenital syndromes, infertility, miscarriage, some cancers and emergency presentations in newborns and adults. At a glance Most human somatic cells contain 46 chromosomes in 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes. A chromosome consists mainly of one long DNA molecule associated with histone and other proteins. Before cell division, each chromosome is copied into two sister chromatids joined at a centromere. Aneuploidy is an abnormal number of individual chromosomes; polyploidy is an extra complete set. Structural abnormalities include deletion, duplication, inversion, translocation, ring chromosome and isochromosome. A karyotype describes chromosome number and large-scale structure; microarray, FISH and sequencing detect smaller or specific changes. Learning outcomes The learner should be able to define a chromosome, explain its parts and packaging, distinguish homologous chromosomes from sister chromatids, describe karyotype notation and cell-division behaviour, classify numerical and structural abnormalities, recognise common clinical patterns and select appropriate first-line investigations. 1. Definition and basic organisation A chromosome is a single, very long DNA molecule packaged with histones and other chromosomal proteins. During interphase, DNA is distributed through the nucleus as chromatin; during cell division, it condenses into visible chromosomes. Each chromosome has a characteristic sequence, size, centromere position and banding pattern. In a diploid human somatic cell, one homologous chromosome is inherited from each parent. Homologues carry the same genes at corresponding loci but can contain different alleles. They are not identical copies. Sister chromatids, by contrast, are near-identical copies produced when one chromosome replicates before mitosis or meiosis. 2. Parts of a chromosome Part Function Clinical significance DNA molecule Carries genes, regulatory elements and non-coding sequences Variants or rearrangements may alter dosage or expression Histones/chromatin Compact DNA and regulate accessibility Epigenetic dysregulation can affect development and cancer Centromere Builds the kinetochore for spindle attachment Errors cause chromosome mis-segregation p arm Short arm (p for petit) Breakpoints are recorded using band notation q arm Long arm (q follows p) Many clinical deletions and translocations are named by q band Telomeres Protect chromosome ends and prevent end-to-end fusion Shortening and telomere disorders cause premature ageing or marrow failure Sister chromatids Replicated copies joined before separation Non-disjunction or chromatid breakage can produce abnormal daughter cells 3. Chromatin and higher-order packaging DNA winds around histone octamers to form nucleosomes. Nucleosomes fold into loops attached to a protein scaffold and occupy chromosome territories within the nucleus. Euchromatin is relatively open and often transcriptionally active; heterochromatin is compact and often less active. Chromosome conformation brings distant enhancers into contact with promoters, so a rearrangement can disturb gene regulation even when no coding sequence is deleted. 4. Human chromosome complement 4.1 Autosomes and sex chromosomes Chromosomes 1–22 are autosomes. The 23rd pair comprises sex chromosomes, conventionally XX or XY, although variations such as XO, XXY and mosaic combinations occur. Sex development depends on chromosome content, gene dosage, gonadal pathways and hormone response; chromosome appearance alone does not define a person’s gender identity. 4.2 Haploid and diploid cells Diploid (2n): most somatic cells contain 46 chromosomes. Haploid (n): mature eggs and sperm contain 23 chromosomes. Fertilisation restores diploidy. Errors during gamete formation or early embryonic divisions can produce aneuploidy or mosaicism. 4.3 Karyotype notation A normal female karyotype is written 46,XX and a normal male karyotype 46,XY. Examples include 47,XX,+21 (an extra chromosome 21), 45,X (Turner syndrome) and 46,XX,t(14;21) (a translocation involving chromosomes 14 and 21). Laboratory reports may include mosaic cell lines and detailed breakpoint notation. 5. Chromosome behaviour in the cell cycle 5.1 Replication During S phase, each chromosome is copied. The two sister chromatids remain connected at the centromere until separation. DNA replication is semi-conservative and includes proofreading and repair; unrepaired breaks or replication errors can become structural variants. 5.2 Mitosis Prophase/prometaphase: chromatin condenses, the nuclear envelope breaks down and spindle fibres attach to kinetochores. Metaphase: chromosomes align at the equator. Anaphase: sister chromatids separate to opposite poles. Telophase and cytokinesis: nuclei reform and two genetically similar daughter cells result. Mitotic errors in somatic cells can produce mosaicism or contribute to cancer. Checkpoint failure allows cells with damaged or abnormal chromosomes to continue dividing. 5.3 Meiosis Meiosis produces haploid gametes through two divisions. Homologous chromosomes pair and exchange segments during prophase I; homologues separate in meiosis I and sister chromatids in meiosis II. Crossing-over increases diversity, but abnormal recombination can create deletions, duplications or translocations. Non-disjunction leaves a gamete with an extra or missing chromosome. 6. Numerical chromosome abnormalities 6.1 Aneuploidy Aneuploidy is loss or gain of one or more individual chromosomes. Trisomy means three copies; monosomy means one copy. Most autosomal monosomies are incompatible with survival, while trisomy 21, 18 and 13 may survive with characteristic clinical patterns. Sex-chromosome aneuploidies often have milder or variable features because of X-inactivation and lower gene density. 6.2 Polyploidy Triploidy (three complete sets) and tetraploidy (four sets) affect every chromosome and usually cause severe developmental abnormalities or early pregnancy loss. Mechanisms include fertilisation by two sperm, failure of meiotic division or failure of early mitotic cytokinesis. 6.3 Mosaicism Mosaicism occurs when a post-zygotic mitotic error creates genetically different cell lines. The phenotype depends on which tissues carry the abnormal line and the proportion of affected cells. A blood karyotype can therefore be normal while another tissue is affected. 7. Structural chromosome abnormalities Abnormality Mechanism Typical consequence Deletion Segment is lost Haploinsufficiency, contiguous-gene syndrome or cancer driver Duplication Segment is repeated Increased gene dosage or altered reading frame Inversion Segment excises, reverses and reinserts Often balanced, but may disrupt a gene or impair meiosis Reciprocal translocation Segments exchange between non-homologous chromosomes Balanced carrier may be well; offspring may inherit an unbalanced complement Robertsonian translocation Long arms of acrocentric chromosomes fuse Carrier has 45 chromosomes but may be phenotypically normal; reproductive risk varies Ring chromosome Both ends break and join in a ring Loss

A gene is a functional unit of heredity encoded in DNA. In modern genetics, a gene is not simply a short stretch that “makes one protein.” A gene includes a transcribed sequence and its regulatory context; it may produce a protein, a functional RNA or several products through alternative processing. Gene structure and regulation explain normal development, inherited disease, cancer, pharmacogenomics and many emergency presentations. At a glance Gene: a DNA sequence, together with its functional regulatory context, that contributes to a heritable product or function. Locus: the physical position of a gene on a chromosome. Allele: one version of a gene or genomic sequence at a locus. Genome: the complete genetic material of an organism. Expression: use of genetic information to produce a functional RNA or protein in a particular cell and context. Clinical point: a DNA variant is not automatically disease-causing; interpretation depends on evidence, inheritance, penetrance, environment and phenotype. Learning outcomes By the end of this page, the learner should be able to define a gene and related terms, describe DNA structure and gene organisation, explain transcription, RNA processing and translation, discuss regulation and epigenetics, classify common variants and inheritance patterns, and interpret basic genetic information in clinical practice. 1. What exactly is a gene? Classical genetics described a gene as a unit that controls an inherited characteristic. Molecular genetics defines it more precisely as a genomic region whose sequence and regulation produce a functional product—usually a protein or a functional RNA—and whose variants can be transmitted through cell division or reproduction. The boundaries of a gene may include promoters, untranslated regions, exons, introns, enhancers and other regulatory elements that can lie some distance away. Not every DNA segment is a gene, and one gene does not always equal one trait. Most clinically important traits result from interactions among multiple genes, regulatory networks and the environment. A single gene can also influence several organ systems (pleiotropy), and the same phenotype can arise from variants in different genes (genetic heterogeneity). 2. Key vocabulary Term Meaning Clinical use Genome All genetic material in a cell or organism Whole-genome testing surveys coding and non-coding regions Chromosome One organised DNA-protein molecule in the nucleus Chromosome number or structure may be abnormal Locus Physical genomic location Describes where a gene or variant lies Allele One sequence version at a locus People usually carry two alleles of autosomal loci Genotype Individual’s genetic constitution at one or more loci For example, two different alleles are heterozygous Phenotype Observable or measurable characteristics Includes symptoms, laboratory values and response to treatment Variant A DNA sequence difference; significance must be interpreted May be benign, pathogenic or of uncertain significance Mutation Older broad term for a sequence change; often replaced by “variant” Use precise language and avoid assuming pathogenicity Haplotype Set of linked alleles inherited together Useful in ancestry, linkage and some disease associations 3. DNA as the genetic material 3.1 Chemical structure DNA is a double-stranded polymer of nucleotides. Each nucleotide contains a deoxyribose sugar, phosphate and one nitrogenous base: adenine (A), thymine (T), cytosine (C) or guanine (G). A pairs with T and C pairs with G through hydrogen bonds. The strands are complementary and antiparallel, allowing one strand to guide accurate copying of the other. 3.2 Packaging DNA wraps around histone proteins to form nucleosomes. Nucleosomes fold into chromatin, which is organised into chromosomes during cell division. Open, relatively accessible euchromatin is generally more transcriptionally active; compact heterochromatin is less accessible, although the relationship is context-dependent. 3.3 Coding and non-coding DNA Protein-coding genes contain exons that contribute to the coding sequence, but exons can also include untranslated regions. Non-coding genes produce functional RNAs such as ribosomal RNA, transfer RNA, microRNA and long non-coding RNA. Regulatory DNA includes promoters, enhancers, silencers, insulators and boundary elements. A regulatory variant can alter expression without changing the protein sequence. 4. Gene organisation Region Function Variant consequence Promoter Near the transcription start site; recruits transcription machinery Reduced or abnormal transcription Enhancer/silencer Regulates expression, sometimes far from the gene Cell-specific or developmental expression change 5′ untranslated region Helps regulate RNA stability and translation Altered translation or RNA processing Exon Retained in mature RNA; may encode protein or untranslated sequence Missense, nonsense, synonymous or splice-related effect Intron Removed from pre-mRNA; contains regulatory information in some genes Splice disruption or cryptic exon inclusion 3′ untranslated region Controls stability, localisation and translation through regulatory elements Abnormal mRNA half-life or protein amount Polyadenylation signal Directs cleavage and poly(A) tail addition Unstable or incorrectly processed RNA 5. From gene to function: the flow of genetic information 5.1 Transcription Transcription factors bind regulatory DNA and recruit RNA polymerase. The enzyme reads the template strand and synthesises a complementary RNA transcript. Transcription is cell-specific: a hepatocyte and a neuron contain essentially the same genome but express different sets of genes. 5.2 RNA processing In eukaryotic cells, the primary transcript is capped at the 5′ end, introns are removed by splicing and a poly(A) tail is added. Alternative splicing allows one gene to produce different mRNA and protein isoforms. Splicing can be altered by variants at canonical splice sites, branch points, enhancers or silencers. 5.3 Translation Ribosomes read mRNA codons and join amino acids into a polypeptide. Transfer RNAs match codons with amino acids; start and stop codons define the open reading frame. The protein then folds, may be cleaved or chemically modified, and is transported to its functional location. 5.4 RNA genes and non-coding function Some genes do not produce proteins. Ribosomal and transfer RNAs are core components of translation; microRNAs can reduce translation or destabilise target mRNAs; long non-coding RNAs influence chromatin and transcription. A variant in a non-coding gene can therefore cause disease even when all protein-coding exons are normal. 6. Regulation of gene expression Transcription factors: proteins that activate or repress sets of genes in response to developmental, hormonal, metabolic or stress signals. Chromatin remodelling: nucleosome repositioning changes DNA accessibility. DNA methylation: often reduces transcription when promoter CpG regions are methylated, although context matters.

Healing is determined by an interaction between biology, mechanics and care. A fracture with an excellent reduction may fail when blood supply is absent or infection is present; a well-vascularised fracture may fail when excessive motion, a large gap or premature loading persists. The same principles apply to skin, soft tissue and bone wounds, although this page emphasises fracture and wound healing relevant to emergency medicine. Core model Successful healing = viable tissue + adequate perfusion + controlled inflammation + appropriate stability + sufficient substrate + effective follow-up. A problem in one domain can be compensated for only partially by the others. For example, rigid fixation cannot rescue devascularised bone, and antibiotics cannot replace drainage or debridement of an abscess. Learning outcomes By the end of this page, the learner should be able to classify factors that influence healing, explain their mechanisms, identify modifiable risks, assess a patient whose wound or fracture is not progressing and construct an emergency plan that protects both biology and mechanics. 1. Local biological factors 1.1 Blood supply and oxygenation Periosteal, endosteal and soft-tissue vessels deliver oxygen, nutrients and progenitor cells. Stripping periosteum, crushing muscle, avulsing skin or injuring a nutrient artery reduces the cellular response. Some regions have naturally vulnerable circulation, including the scaphoid waist/proximal pole, talar neck, femoral neck and selected tibial areas. A fracture there can develop avascular necrosis even with good alignment. Shock, hypoxaemia, severe anaemia, vascular injury, compartment syndrome, smoking-related vasoconstriction and excessive swelling further reduce tissue oxygen. 1.2 Soft-tissue injury Skin, muscle and fascia provide blood vessels and cells for callus. High-energy crush, degloving, thermal injury and repeated surgical exposure enlarge the zone of injury. A small wound may therefore conceal a large area of devitalised tissue. 1.3 Contamination and infection Soil, saliva, faeces, water, foreign bodies and devitalised tissue increase microbial load. Bacteria compete for oxygen, produce toxins and biofilm, and sustain inflammatory cytokines that inhibit osteoblasts. Infection can cause osteomyelitis, implant loosening, sinus formation and non-union. Early source control is more important than relying on antibiotics alone. 1.4 Bone and fracture characteristics Local feature Why it matters Possible effect Comminution or segmental fracture Small fragments may lose their blood supply; reconstruction is mechanically demanding Delayed union, infection or non-union Large gap or bone loss Progenitor cells and vessels must bridge a greater distance Persistent fibrous tissue or atrophic non-union Interposed muscle, periosteum or foreign body Prevents contact and may harbour bacteria Failure of reduction or union Intra-articular extension Cartilage has limited regenerative capacity and stiffness develops easily Post-traumatic arthritis and impaired function Pathological bone Tumour, infection or metabolic disease changes matrix and blood supply Unusual fracture pattern and poor healing 2. Mechanical factors 2.1 Stability and interfragmentary strain Healing tissue tolerates only a limited amount of deformation. Controlled relative motion can promote callus, but excessive shear, rotation or distraction keeps the tissue in a fibrous state. Primary/direct healing needs very rigid, near-anatomic fixation; secondary healing needs relative stability with a manageable strain environment. 2.2 Alignment and contact Malrotation, severe angulation, shortening and translation alter load distribution and may produce malunion even when a bridge eventually forms. Excessive compression can compromise microcirculation, whereas a large gap may exceed the capacity of callus to bridge. Joint incongruity increases focal cartilage stress and long-term arthritis risk. 2.3 Fixation and loading Loose screws, plate failure, inadequate cast moulding, broken external-fixator components or premature weight-bearing can create a mechanically unstable non-union. Conversely, prolonged complete immobilisation can cause stiffness, muscle wasting, thrombosis and delayed functional recovery. The correct loading plan is fracture- and fixation-specific and should be documented by the treating team. 3. Patient-related systemic factors 3.1 Age and hormonal state Children generally have thicker periosteum, more active osteoprogenitor cells and better remodelling. Older adults may have reduced osteoblast activity, osteoporosis, vascular disease, lower activity and polypharmacy. Menopause, hypogonadism and endocrine disorders can affect bone turnover. 3.2 Diabetes mellitus Hyperglycaemia impairs leukocyte function, angiogenesis and collagen formation and increases infection risk. Peripheral neuropathy can delay presentation and allow excessive loading. Optimise glucose safely, inspect skin and feet, and do not assume a painless wound is a healthy wound. 3.3 Vascular and respiratory disease Peripheral arterial disease reduces oxygen and nutrient delivery. Heart failure, chronic lung disease, sleep-disordered breathing and severe anaemia can reduce oxygen availability. Vasoconstrictive drugs or profound hypotension may worsen an already marginal flap or fracture fragment. 3.4 Renal, hepatic and endocrine disease Chronic kidney disease disturbs calcium-phosphate and parathyroid hormone balance and may cause renal osteodystrophy. Liver disease affects protein synthesis and nutrition. Thyroid, parathyroid and sex-hormone disorders alter bone turnover. Consider these conditions when a low-energy fracture or delayed union is unexplained. 3.5 Immune status and infection HIV with advanced immunosuppression, malignancy, corticosteroid therapy, chemotherapy, severe sepsis and other immunodeficiency states increase infection risk and may reduce tissue repair. Review vaccination, fever, recurrent infections and medication history without stigmatising the patient. 4. Nutrition and metabolic substrates Repair requires energy, amino acids, essential fatty acids, iron, zinc, copper, vitamin C, vitamin D, calcium and phosphate. Deficiency may be overt or hidden by chronic illness, alcohol use, food insecurity or prolonged hospitalisation. Nutrient/metabolic issue Role in healing Clues and response Protein/energy Collagen, enzymes, immune cells and muscle Weight loss, low intake, pressure injury; dietitian support and adequate protein Vitamin C Collagen hydroxylation and antioxidant function Poor diet, bruising or gum disease; correct documented deficiency Vitamin D/calcium/phosphate Mineralisation and skeletal metabolism Low sunlight, malabsorption, renal disease; test and replace according to local guidance Iron/anaemia Oxygen transport and cellular metabolism Fatigue, pallor, blood loss; identify and treat the cause Zinc/copper Enzymes, immunity and connective tissue Malabsorption or prolonged restrictive diet; avoid indiscriminate megadoses Routine high-dose supplements do not substitute for diagnosis and may be harmful. Ask about alcohol, restrictive diets, vomiting, diarrhoea and access to food, and use local nutrition pathways. 5. Smoking, alcohol and substance exposure Smoking and nicotine Nicotine causes vasoconstriction and can suppress osteoblast differentiation; carbon monoxide reduces oxygen delivery and smoke-related toxins impair immune function. Smoking increases wound