mutation

If a mutation causes the gene to stop functioning altogether or vastly
alters normal function, it’s considered a loss-of-function mutation. All nonsense
mutations are loss-of-function mutations, but not all loss-of-function
mutations are the result of nonsense mutations. The usefulness of the protein
made from a particular gene can be lost even when no stop codon
has been added prematurely. Insertions and deletions are often loss-offunction
mutations because they cause frameshifts (Chapter 9 explains
how the genetic code is read in frames). Frameshifts cause an entirely
new set of amino acids to be put together from the new set of instructions.
Most of the time, these new proteins are useless and nonfunctional.
Loss-of-function mutations are usually recessive because the normal,
unmutated allele is still producing product, usually enough to compensate
for the mutated allele. Loss-of-function mutations are only detected when
a person is homozygous for the mutation and is making no functional gene
product at all.