Gene therapy is a newer approach to treating diseases based on modifying a person’s genes toward a therapeutic goal. Gene therapy has been targeted towards treating lethal and disabling diseases. It also has potential of preventing diseases. This method of treatment is still in its infancy.
It can be said that the gene that we inherit from our parents influence practically every disease. A composite of approximately 150,000 individual genes constitutes a human being. Several years ago, an international effort was launched to identify every single human gene. This effort, called the Human Genome Project. Variation in the structure of a person’s genes collectively helps define us as individuals such as how tall we are to what color our eyes are supposed to have. Some of this genetic ‘miscalculations’ unfortunately leads to the development of disease. The genetics of many diseases are passed from one generation to the next by inheriting a single gene. An example is Huntingdon’s disease. Many other diseases and traits are influenced by a collection of genes.
The premise of gene therapy is to treat the disease at its root. There are two types of Gene Theapy. Somatic Gene Therapy and  Germline Gene Therapy.
Somatic gene therapy involves the manipulation of gene expression in cells that will be corrective to the patient but not inherited to the next generation.
Germline gene therapy, this involves the genetic modification of germ cells that will pass the change on to the next generation.
It is the Somatic Gene therapy that is being mainly investigated throughout the world. The work on Germline gene therapy is restricted due to technical and ethical reasons.
To deliver genetic material to the appropriate cells of the patient in a way that is specific, efficient and safe, gene delivery vehicles called vectors have been created. The vectors being used are modified and attenuated viruses. The virus is modified in such a way that its disease causing component is removed and in its place gene are inserted. Synthetic vectors are also being used formed of complexes of DNA, Protein and Lipids.
The first human trials of Gene therapy began in 1990. Many types of diseases are currently being investigated as candidates for gene therapy including cardiovascular diseases, cholesterol lowering therapy, infectious diseases such as AIDS, and cancer.
Gene therapy can be used not only in treating genetic diseases but also to deliver specific proteins. By placing genes in laboratory-cultured organisms that produce the proteins coded by those genes. Examples of such manufactured proteins include insulin, growth hormone, and erythropoietin, all of which must be injected frequently into the patient.
In hemophilia treatments, a gene-carrying vector could be injected into a muscle, prompting the muscle cells to produce Factor IX and thus prevent bleeding. This method would end the need for injections of Factor IX — a derivative of pooled blood products and a potential source of HIV and hepatitis infection. In gene therapies such as these, the introduced gene is always “on” so the protein is always being produced, possibly even in instances when it isn’t needed.

For the first time, scientists have mapped virtually an entire human chromosome, one of the chains of molecules that bear the genetic recipe for human life. The achievement was announced on Wednesday 1st Dec ’99. It was an important step for the $3 billion Human Genome Project, which is attempting to detail the tens of thousands of genes that carry instructions for everything in a human from brain function to hair color to foot size.
This is probably the most important scientific effort that mankind has ever mounted. That includes splitting the atom and going to the moon. In laying out the chemical instructions for life, scientists believe they are in the early stages of revolutionizing the study of human development and medicine. Already, researchers have begun testing several biological therapies that replace faulty genes or correct their misfirings to make cells work correctly. Such therapies, if they can be made reliable, would bring a more precise way to treat diseases without the sometimes debilitating side effects of conventional drugs.
The defects in the genes along the chromosome contribute to heart defects, immune system disorders, cancers, schizophrenia and mental retardation.

The human genetic pattern, or genome, is a biological map laying out the sequence of 3 billion pairs of chemicals that make up the DNA in each cell. All human DNA is contained within 23 pairs of chromosomes.

Genes are arrayed along chromosomes, the rod-shaped bodies inside the nucleus of a cell. Proteins and other compounds carry out the instructions of genes. Inside the chromosomes, genetic material is linked along tightly coiled strands of the master molecule DNA, which twists like a spiral ladder. Each rung is built with pairs of four chemical bases ordered in different numbers and combinations to form genes.

Mapping a gene is only an early step in understanding the gene’s function and how it might contribute to a particular disease.

Human Genome 
Genome Sequencing helps diagnosing rare disorder
Genome Editing