To function correctly, each cell depends on thousands of proteins to do their jobs in the right places at the right times. If a mutation in a gene changes the order of amino acids in the protein produced from that gene, that can change the protein’s function, causing it to malfunction or to be missing entirely. When a mutation alters a protein that plays a critical role in the body, it can disrupt normal development or cause a medical condition. A condition caused by mutations in one or more genes is called a genetic disorder.
In some cases, gene mutations are so severe that they prevent an embryo from surviving until birth. These changes occur in genes that are essential for development, and often disrupt the development of an embryo in its earliest stages. Because these mutations have very serious effects, they are incompatible with life.
It is important to note that genes themselves do not cause disease—genetic disorders are caused by mutations that make a gene function improperly. For example, when people say that someone has “the cystic fibrosis gene,” they are usually referring to a mutated version of the CFTR gene, which causes the disease. All people, including those without cystic fibrosis, have a version of the CFTR gene. These different versions of a gene are called alleles. Someone who has cystic fibrosis will have alleles of the CFTR gene that contain mutations which lead to the genetic disorder. Someone who does not have cystic fibrosis will have alleles of the CFTR gene that do not contain mutations.