As of now, gene therapy has only been used to single-gene disorders. These kind of disorders are only caused when you inherit both sets of gene from carrier parents. Single-gene disorders are recessive, which means you need to have both sets of genes with the disorder, one from each parent, to have the disorder (Fig 1). So, having both carrier parents, each with one set of the incorrect gene, will give the child 25% of having the disorder, 50% to be a carrier - people who have the gene but has a dominant gene which masks the incorrect gene - and 25% of not having the gene at all.
Adeinosine Deaminase Deficiency
One kind of genetic disorder that has already been corrected by gene therapy is ADA deficiency. People with Adenosine deaminase deficiency do not produce ADA which is necessary for the body to break down a toxic with in the body. This toxic kills off B and T cell which are a important part of the bodies immune system. As shown in the diagram, normal people with ADA are able to convert deoxyadenosine which is toxic to the body to deoxyinosine which is non-toxic. People who have ADA deficiency cannot covert the toxic which lead to high levels of deoxyadenosine (Fig 2).
Cystic Fibrosis
Cystic fibrosis is a genetic disorder that affects the respiratory and digestive systems. Mucus naturally builds up on the inner lining of digestive and respiratory systems. Normal people are able to produce a protein, this protein move sodium chloride in and out of cells, thus reducing the thickness of the mucus. People with cystic fibrosis inherit a defective gene on 7th chromosome called CFTR (cystic fibrosis transmembrane conductance regulator) which does not allow the protein to function properly. The build up of mucus clogs airways allowing bacteria to infect such areas (Fig 3). In the digestive system, the mucus blocks ducts in the pancreas, so enzymes which breaks down food are not able to ingest all the nutrition from food needed to grow normally.