Single-gene disorders, also called monogenic disorders, are defects caused by a mutant allele of a single gene that results in the functional loss of a protein. Single-gene disorders are generally easy to trace because they follow the rules of classical Medelian genetics. Single-gene disorders can be autosomal or X-linked, dominant or recessive and usually become evident by studying a particular family's pedigree. Common single-gene disorders include sickle cell anemia, familial hypercholesterolemia, Duchenne muscular dystrophy, and cystic fibrosis. Some speech and language disorders are also caused by the interference of a single gene.

Mutations in DNA (deoxyribonucleic acid) arise because of a spontaneous chemical change that results in a substitution, deletion, or insertion of a nucleotide base pair. As a result, many mutations cause physical or mental disorders. Sickle cell anemia is caused by a nucleotide base substitution that results in a defective hemoglobin molecule. Because sickle...