Loss of function mutations

Loss of function mutations result in reduced or absent function
of the gene product. This type of mutation is the most
common, and generally results in a recessive phenotype, in
which heterozygotes with 50% of normal gene activity are
unaffected, and only homozygotes with complete loss of
function are clinically affected. Occasionally, loss of function
mutations may have a dominant effect. Heterozygosity for
chromosomal deletions usually causes an abnormal phenotype
and this is probably due to haploinsufficiency of a number of
genes.