How Are Diseases Inherited?

Genetic Mutation

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Many diseases and disorders occur as a result of alterations or mutations in a particular gene, and some of these mutations can be passed on to future generations. Sometimes this inheritance is straightforward, while other times additional genetic changes or environmental factors also need to be present for a particular disease to develop. Gregor Mendel studied pleiotrophy, the expression of multiple traits by a single gene, which can cause certain genetic diseases.

Autosomal Recessive Inheritance

Some diseases or traits require two mutated copies of a specific gene in order to develop — one from each parent. In other words, both parents must have the particular gene and pass it on in order for their child to possibly be affected. If the child receives only one copy of a recessive mutated gene, then they are called a carrier; they will not develop the disease but can pass it on to their children. When both parents are unaffected carriers (meaning that each has only one copy of a particular recessive faulty gene), then there is a 25% chance that their child will inherit the faulty gene copy from both parents and be affected by or predisposed to develop the condition or disease, and a 50% chance the child will inherit only one copy of the mutated gene (become a carrier).

Examples of diseases inherited in autosomal recessive fashion include cystic fibrosis, hemochromatosis, and Tay-Sachs disease. It is possible in some cases to test an individual to determine whether they are a carrier of a specific faulty gene.

Autosomal Dominant Inheritance

Sometimes, only one parent has to pass on a mutated gene in order for their child to inherit a risk for a specific disease. This does not always mean that the disease will develop, but the increased risk for that disease is there.

Example of diseases that can be inherited through autosomal dominance includes Huntington's disease, achondroplasia (a form of dwarfism) and familial adenomatous polyposis (FAP), a disorder characterized by colon polyps and a predisposition to colon cancer.

X-Linked Inheritance

Many diseases and disorders that are associated with the X (female) chromosome are more likely to be inherited by men than women. This is because females inherit two X chromosomes (one from each of their parents), while males inherit one X chromosome (from their mother) and one Y chromosome (from their father). A man who inherits one copy of a recessive mutated gene on his X chromosome will develop that trait because he has no additional copies of that gene; while a woman would have to inherit the recessive mutation from both parents in order to develop the disease or trait. This type of disorder ultimately affects almost twice as many females as males (although many solely as carriers), however, because an affected father can never pass an X-linked trait to his sons, but does pass it to all of his daughters, while an affected mother passes an X-linked trait to half of her daughters and half of her sons.

Diseases caused by mutations on the X chromosome, called X-linked diseases, including hemophilia (a blood clotting disorder) and color blindness.

Mitochondrial Inheritance

The mitochondria in our cells have their own DNA, which is separate from the rest of the cell's DNA. Sometimes diseases occur when numerous copies of mitochondrial DNA within a cell are impaired or do not work properly. Almost all mitochondrial DNA is carried in the egg, so disease genes carried on mitochondrial DNA can be passed only from mother to child. Thus, this pattern of inheritance is often called maternal inheritance.

An inherited mutation doesn't always mean the disease or disorder will develop. In some cases, a faulty gene will not be expressed unless other environmental factors or changes in other genes are also present. In these cases, the individual has inherited an increased risk for the disease or disorder, but may not ever develop the disease. The inherited form of breast cancer is one such example. Inheritance of the BRCA1 or BRCA2 gene does drastically increase a woman's chance of developing breast cancer (from about 12% to about 55–65% for BRCA1 and to about 45% for BRCA2), but some women who inherit a harmful BRCA1 or BRCA2 mutation will still never develop breast or ovarian cancer.

It is also possible to develop a disease or disorder due to a genetic mutation that is not inherited. In this case, the genetic mutation is somatic, meaning that the genes changed during your lifetime.