A cell is regarded as the basic unit of life. Located in each cell is a set of 46 chromosomes, 23 of which we receive from the mother and 23 from the father at conception. These chromosomes contain "genes" that act as the blueprint for the production of specific proteins, which code for our appearance, different facets of our personality, our health etc.. Each gene is composed of DNA, which carries the hereditary information in an individual.

Each of us carries about half a dozen of defective genes. However, most of us do not suffer any harmful effects from our defective genes because we carry two copies of nearly all genes, one derived from the mother, and the other from the father. In most cases, the potentially harmful defective gene is "recessive" and the normal copy of the gene is enough to avoid all the symptoms of the disease...