A genetic disorder or condition is the result of changes, or mutations, in an individual’s DNA. A mutation is a change in the letters (DNA sequence) that make up a gene. This is sometimes referred to as a “spelling” mistake. Most genes code for proteins, the molecules that carry out most of the work, perform most life functions, and make up the majority of cellular structures. Some genes code for other structural and regulatory molecules (non-coding RNA). When a gene is mutated so that its protein or molecule product can no longer carry out its normal function, a disorder can result.
Genetic diseases can be inherited because they are mutations in the germ cells of the body - sperm and egg cells involved in passing genetic information from parents to offspring. Genetic diseases can also result from changes in DNA in somatic cells, or cells in the body that are not germ cells (such as cells in the lungs, skin cells, or cells in the bone marrow).
Some genetic diseases are called Mendelian disorders—they are caused by mutations that occur in the DNA sequence of a single gene. These are usually rare diseases; some examples are Huntington’s disease, sickle cell disease and cystic fibrosis. Many genetic diseases are multifactorial—they are caused by mutations in several genes but are also influenced by environmental factors. Some examples of these are heart disease, cancer, and diabetes.