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How Diseases are Inherited

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Multi-generation family lying down in a row, smiling
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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.

Autosomal Recessive

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 have the disease or trait. If the child receives only one copy of a recessive mutated gene, then they are called carriers; 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 1 chance in 4 that their child will inherit the faulty gene copy from both parents and be affected by or predisposed to develop the condition or disease.

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

Autosomal Dominant

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 the disease will always develop, but the increased risk for that disease is there.

Example of diseases that can be inherited through autosomal dominance include 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

Many diseases and disorders that are associated with the X 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, include 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.

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