Dominant mutations can be categorized according to whether they increase or
decrease the overall activity of a gene or gene product. Although a loss-of-
function mutation (a mutation that inactivates the gene product) is usually
recessive, for some genes, one dose of the normal gene product, encoded by the
normal allele, is not sufficient to produce a normal phenotype. In this case,
a loss-of-function mutation in the gene will be dominant, and the gene is said
to be haploinsufficient. A second category of dominant mutation is the gain-
of-function mutation, which results in a new activity or increased activity or
expression of a gene or gene product. The gene therapy technique currently
being used in clinical trials involves the "addition" to somatic cells of a
normal copy of a gene. In other words, a normal copy of the gene is inserted
into the genome of the mutant somatic cell, but the mutated copy of the gene
is not removed or replaced. Will this strategy work for either of the two
aforementioned types of dominant mutations?
