Inherited diseases are caused by abnormal genetic material, like individual genes and chromosomes, passed on to a child from one or both parents. These types of diseases are also commonly referred to as genetic disorders, inherited genetic diseases, or hereditary diseases, and can have varying symptoms and degrees of severity. Some common inherited diseases are cystic fibrosis, a degenerative condition that causes disability and often early death; sickle-cell anemia, which is a blood disorder that can vary in severity; and Huntington's disease, which causes degeneration of the brain and nervous system. Often, inherited diseases cannot be cured, though symptoms can sometimes be managed with medication and other forms of medical care. Gene therapy, a mostly experimental type of treatment, is being researched as a way to cure some genetic disorders by removing or altering defective genes.
The genetic material of every human being is made up of a vast number of individual genes that affect a wide variety of traits and abilities, including physical appearance and cognitive function. In this genetic material, there are almost always two copies of every gene: one inherited from each parent. A hereditary condition, such as an inherited disease, is passed on to a child if either one or both parents have an abnormal gene.
Some inherited diseases, for example cystic fibrosis, are called autosomal recessive disorders, and require both genes in the genetic pair to be abnormal for the disease to develop. A person who carries only one abnormal gene for a recessive disorder will not develop the disease, but can pass it on to the next generation. Other inherited diseases, like Huntington's disease, are called autosomal dominant disorders and require only one of the genes in a pair to be defective for the disease to develop.
Genetics is not always the only determining factor when it comes to inherited diseases. For example, in the case of some inherited disorders of the autosomal dominant type, not everyone who inherits a defective gene will develop the disease. There are also examples of inherited diseases where the defective genetic material by itself does not cause disease, but can do so in conjunction with various environmental factors. A related term is congenital disease, which refers to a condition that develops at or before birth, but does not necessarily have to be inherited. One example is Down's syndrome, a genetic disorder that is not inherited, but instead caused by an extra chromosome forming at an early stage of fetal development.