Single gene disease: Any genetic disorder caused by a change affecting only one gene.

There are thousands of single-gene diseases including achondroplasia, cystic fibrosis, hemophilia, Huntington disease, muscular dystrophy, and sickle cell disease .

Single-gene diseases usually follow simple Mendelian patterns of inheritance -- autosomal dominant, an autosomal recessive, or an X-linked.

Single gene diseases are as opposed to polygenic diseases which are caused by a combination of two or more genes.