During the past 10 years, enormous strides have been made both in our ability to detect abnormal genes and in the number of genes that can be analyzed. The introduction of the polymerase chain reaction (PCR), a method that can enrich a region of DNA by a million-fold, enables gene diagnosis to be performed within hours instead of weeks and is moving DNA testing increasingly from the research bench to the hospital laboratory. The human genome is estimated to have about 100 000 genes, and almost 5000 of these genes have been categorized. The list of identified genes now includes those responsible for a number of relatively common genetic disorders, including cystic fibrosis, Duchenne muscular dystrophy, hemophilia A, sickle cell disease, and neurofibromatosis. At the present time, more than 200 genetic diseases can be diagnosed using DNA techniques. A sample of these is shown in the Table. Inheritance of Abnormal Genes Because individuals inherit one set of chromosomes from each parent, there are two genes at each locus, with the exception of genes on the X chromosome because there is only one X chromosome in males. Disorders produced by an abnormality in only one gene are termed dominant (eg, neurofibromatosis), whereas disorders that occur only when both genes are affected are termed recessive (eg, cystic fibrosis).