most genetic disorders are covered by single genes inherited in mendelian fashion
Mendelian inheritance A Mendelian trait is one that is controlled by a single locus in an inheritance pattern. In such cases, a mutation in a single gene can cause a disease that is inherited according to Mendel's laws. Examples include sickle-cell anemia, Tay-Sachs disease, cystic fibrosis and xeroderma pigmentosa. A disease controlled by a single gene contrasts with a multi-factorial disease, like arthritis, which is affected by several loci (and the environment) as well as those diseases inherited in a non-Mendelian fashion.[citation needed]
Huntington's disease HD is typically inherited from a person's parents, although up to 10% of cases are due to a new mutation.[1] The disease is caused by an autosomal dominant mutation in either of an individual's two copies of a gene called Huntingtin.[4] This means a child of an affected person typically has a 50% chance of inheriting the disease.[4] The Huntingtin gene provides the genetic information for a protein that is also called "huntingtin".[1] Expansion of CAG (cytosine-adenine-guanine) triplet repeats in the gene coding for the Huntingtin protein results in an abnormal protein, which gradually damages cells in the brain, through mechanisms that are not fully understood.[4] Diagnosis is by genetic testing, which can be carried out at any time, regardless of whether or not symptoms are present.[5] This fact raises several ethical debates: the age at which an individual is considered mature enough to choose testing; whether parents have the right to have their children tested; and managing confidentiality and disclosure of test results.[2]