Inherited diseases are health conditions caused wholly or partly by genetic variants that are transmitted from biological parents to their children. The relevant variants are usually germline variants, meaning they are present in an egg or sperm and can therefore enter the genetic makeup of the child. Inherited diseases are a subset of genetic conditions; not every genetic disorder is inherited because some genetic changes arise spontaneously during development or are acquired during a person's lifetime. Inherited disorders can result from variants in a single gene, chromosome abnormalities, mitochondrial DNA changes or combinations of genetic factors. Their manifestations range from childhood developmental disorders to conditions that first become apparent in adulthood.
Single-gene inherited disorders are caused primarily by pathogenic variants in one gene. Their inheritance pattern depends on the gene and biological mechanism involved. In autosomal dominant inheritance, one pathogenic copy can be sufficient to cause the condition, whereas autosomal recessive disorders generally require pathogenic variants in both copies of the relevant gene. X-linked conditions involve genes on the X chromosome and have distinctive inheritance patterns because males and females have different sex-chromosome complements. Mitochondrial disorders can be transmitted through mitochondrial DNA, which is ordinarily inherited from the mother. These patterns allow clinicians to estimate recurrence risks, although real families can show complexities caused by penetrance, new variants, mosaicism and other factors.
Not all inherited diseases are caused by changes in a single gene. Chromosomal disorders can result from an abnormal number of chromosomes or structural rearrangements involving large DNA segments. Down syndrome, for example, usually results from an additional copy of chromosome 21. Structural chromosome abnormalities can involve deletions, duplications, inversions or translocations. Some chromosomal abnormalities are inherited, while others arise as new events during the formation of eggs or sperm or during early embryonic development. Cancer cells can also acquire chromosomal abnormalities, but these somatic changes are generally not inherited by a person's children. This distinction between germline and somatic chromosome changes is fundamental to understanding heredity.
Many inherited diseases follow recognizable family patterns, but a condition can still be genetic even when there is no previous known family history. A pathogenic variant may arise newly in an affected person, parents may be unaffected carriers of a recessive condition, relatives may have died before symptoms became apparent, or the condition may have variable penetrance. Families can also be small, making patterns difficult to recognise statistically.
Conversely, a disease appearing in several relatives does not automatically prove that it is genetic because relatives share environmental and lifestyle factors as well as DNA. Genetic assessment therefore combines the medical history of the individual with a detailed family history and, where appropriate, laboratory testing.
The effects of inherited genetic variants vary considerably. Some pathogenic variants cause disease with high penetrance, while others increase susceptibility without guaranteeing that disease will occur. Variable expressivity means that people with the same disease-associated genetic change can experience different symptoms or severity. Modifier genes, environmental exposures, age, sex and chance can influence the final phenotype. This explains why two relatives carrying the same pathogenic variant may not have identical clinical presentations. It also explains why genetic testing can identify an inherited risk without providing a precise prediction of an individual's future health. A genetic condition therefore describes a biological predisposition or disease mechanism, while its clinical expression can remain highly individual.
Diagnosis may involve clinical examination, laboratory investigations, family-history analysis and genetic testing. Depending on the suspected condition, testing may analyse a single gene, multiple genes, chromosomes, mitochondrial DNA or the whole genome or exome. Some inherited diseases are diagnosed by a characteristic combination of clinical findings and biochemical abnormalities rather than by genetic testing alone. A confirmed molecular diagnosis can clarify prognosis, identify appropriate surveillance, inform treatment and determine whether relatives should be offered testing. However, a negative genetic test does not always exclude a genetic disorder because current technologies may not detect every pathogenic mechanism and some disease-causing variants remain unknown.
Management depends on the particular disorder. Some inherited diseases have highly effective disease-specific treatments, while others are managed by preventing complications and controlling symptoms. Examples include dietary treatment for certain metabolic disorders, replacement of deficient substances, medicines targeting particular molecular pathways, surgery for structural complications and specialised monitoring. Advances in gene therapy and genome-based medicine are providing treatments for some conditions caused by specific genetic abnormalities, although these approaches are not applicable to every disorder. Genetic diagnosis can also guide preventive surveillance, such as increased screening for cancers associated with inherited susceptibility. Treatment therefore does not necessarily mean eliminating the underlying genetic variant; it may instead modify its consequences.
Inherited diseases have implications beyond the affected individual because relatives may share the same genetic variant. Genetic counselling can explain inheritance patterns, recurrence probabilities, testing options and reproductive considerations. Cascade testing can sometimes identify relatives who carry a pathogenic variant before they develop symptoms, allowing earlier surveillance or preventive measures where effective interventions exist. Importantly, having an inherited disease is not the same as having a condition that is contagious, and a genetic condition cannot be acquired through ordinary contact. Nor does inheritance imply inevitability in every case. The key concepts are transmission of genetic information, variable expression and probability, with the precise implications determined by the particular disorder and genetic mechanism.
