Gene therapy is the process of altering defective genes in a person with modified genes. This is presently possible to do using two methods. One is called Germ Line Gene Therapy. This involves modification of genes in the germ cells which are the eggs and the sperms. This can only be done in children yet to be born and not those who already have been. The benefits of this kind of genetic modifications would be carried over generations as the person reproduces.
The other method is called Somatic Gene Therapy where the altered genes are transferred into the somatic cells of patients. This therapy affects only the person who is being treated and is not passed on to future generations.