Types of mutation

In a few genetic diseases, all affected individuals have the same
mutation. In sickle cell disease, for example, all mutant genes
have a single base substitution, changing the sixth codon of the
beta-globin gene from GAG to GTG, resulting in the
substitution of valine for glutamic acid. In Huntington disease,
all affected individuals have an expansion of a CAG
trinucleotide repeat expansion. The majority of mendelian
disorders are, however, due to many different mutations in a
single gene. In some cases, one or more mutations are
particularly frequent. In cystic fibrosis, for example, over
700 mutations have been described, but one particular
mutation, F508, accounts for about 70% of all cases in
northern Europeans. In many conditions, the range of
mutations observed is very variable. In DMD, for example,
mutations include deletions, duplications and point mutations.