Along with environmental influences, genes and processes inside the body combine to do more than just determine eye and hair color and other traits inherited from our parents. For example, genes ensure that we have two hands and can use them to do things, like play the piano. Genes alone are not all-powerful. Most genes can do little until spurred on by other substances. Each of these messages is used to build a certain protein.
Faulty proteins can lead to cell malfunction, disease, and death.
Huntington's disease are single-gene disorders. If a person inherits the gene that causes one of these disorders, he or she surely will get the disease, unless it is prevented by other means. AD, on the other hand, is not a single-gene disorder. More than one gene mutation can cause AD, and genes on multiple chromosomes are involved. The two basic types of AD are familial and sporadic.