SINGLE GENE MUTATIONS
Many more human diseases result from single gene mutations than from chromosome abnormalities. These include sickle-cell anemia, cystic fibrosis, Huntington disease, phenylketonuria (PKU), and alkaptonuria. These disorders, referred to as inborn errors of metabolism, result from mutations in a single gene, which causes the absence of, or incorrect form of, the protein for which that gene codes.

These diseases usually are inherited as autosomal recessive traits. The heterozygous parents each have one normal copy of the gene and one mutated, nonfunctional copy. In the heterozygote, the cell's normal gene compensates by making more protein. The homozygous recessive individuals, however, exhibit the disorder because both gene copies are nonfunctional and the correct gene product is missing.