A genetic defect occurs when a gene fails to express a certain function or produce a particular protein. Such a defect may cause or be a contributing factor to a debilitating disease or illness. The defect occurs when DNA is miscopied, resulting in what geneticists call a mutation. It should be noted that virtually every human being carries a small number of mutations, but they usually are not expressed in a deleterious manner.

Mutations can be passed from parents to offspring. If only one parent carries a mutated gene it will usually be overridden by a second copy of the gene. If both parents carry the mutated genes, offspring are at risk of being affected. For example, if one parent carries the recessive gene for cystic fibrosis but the other parent does not, there is no risk that offspring will be afflicted with the disease. If both parents carry the recessive gene, however, there is a one-in-four chance that they will...