Some genetic conditions are caused by mutations in a single gene. These conditions are usually inherited in one of several straightforward patterns, depending on the gene involved:
Patterns of inheritance
||One mutated copy of the gene in each cell is sufficient for a person to be affected by an autosomal dominant disorder. Each affected person usually has one affected parent (illustration). Autosomal dominant disorders tend to occur in every generation of an affected family.
||Huntington disease, neurofibromatosis type 1
||Two mutated copies of the gene are present in each cell when a person has an autosomal recessive disorder. An affected person usually has unaffected parents who each carry a single copy of the mutated gene (and are referred to as carriers) (illustration). Autosomal recessive disorders are typically not seen in every generation of an affected family.
||cystic fibrosis, sickle cell disease
||X-linked dominant disorders are caused by mutations in genes on the X chromosome. Females are more frequently affected than males, and the chance of passing on an X-linked dominant disorder differs between men (illustration) and women (illustration). Families with an X-linked dominant disorder often have both affected males and affected females in each generation. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).
||fragile X syndrome
||X-linked recessive disorders are also caused by mutations in genes on the X chromosome. Males are more frequently affected than females, and the chance of passing on the disorder differs between men (illustration) and women (illustration). Families with an X-linked recessive disorder often have affected males, but rarely affected females, in each generation. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).
||hemophilia, Fabry disease
||In codominant inheritance, two different versions (alleles) of a gene can be expressed, and each version makes a slightly different protein (illustration). Both alleles influence the genetic trait or determine the characteristics of the genetic condition.
||ABO blood group, alpha-1 antitrypsin deficiency
||This type of inheritance, also known as maternal inheritance, applies to genes in mitochondrial DNA. Mitochondria, which are structures in each cell that convert molecules into energy, each contain a small amount of DNA. Because only egg cells contribute mitochondria to the developing embryo, only females can pass on mitochondrial mutations to their children (illustration). Disorders resulting from mutations in mitochondrial DNA can appear in every generation of a family and can affect both males and females, but fathers do not pass these disorders to their children.
||Leber hereditary optic neuropathy (LHON)
Many other disorders are caused by a combination of the effects of multiple genes or by interactions between genes and the environment. Such disorders are more difficult to analyze because their genetic causes are often unclear, and they do not follow the patterns of inheritance described above. Examples of conditions caused by multiple genes or gene/environment interactions include heart disease, diabetes, schizophrenia, and certain types of cancer. For more information, please see What are complex or multifactorial disorders?.
Disorders caused by changes in the number or structure of chromosomes do not follow the straightforward patterns of inheritance listed above. To read about how chromosomal conditions occur, please see Are chromosomal disorders inherited?.
Other genetic factors can also influence how a disorder is inherited: What are genomic imprinting and uniparental disomy?
For more information about inheritance patterns:
Resources related to heredity/inheritance patterns and Mendelian inheritance are available from GeneEd.
The Centre for Genetics Education provides information about each of the inheritance patterns outlined above:
EuroGentest also offers explanations of Mendelian inheritance patterns:
Additional information about inheritance patterns is available from The Merck Manual.
The National Genetics and Genomics Education Centre of the National Health Service (UK) provides explanations of various forms of genetic inheritance.
Next: If a genetic disorder runs in my family, what are the chances that my children will have the condition?