Pharmacogenetics is the special areas of biochemical genetics that deals with variation in drug response and the contribution of genetics to such variation. In broad terms, pharmacogenetics can be said to encompass any genetically determined variation in response to drugs: for instance, the effect of barbiturates in precipitating attacks of porphyria in people with the gene for acute intermittent porphyria or the effect of alcohol use by pregnant woman on the incidence of fetal alcohol syndrome. In a narrower sense, pharmacogenetics can be restricted to those genetic variations that are revealed only by response to drugs or other chemicals.
The origin of polymorphisms by which they are maintained pose a problem. They obviously have not developed in response to drugs, since they antedate the drugs concerned. The handling of and the response to drugs require many specific bio-chemical reactions, and the enzymes involved may participate in the metabolism of ordinary food substances.
Recognizing that there is normal variation in response to drugs, pharmacologists define the "potency" of a drug by the dose that produces a given effect in 50 percent of the population. For genetic traits, continuous variation is usually best explained on the basis of multi factorial inheritance or by a combination of genetic and environmental factors. But response to drugs can also show discontinuous variation, with sharp distinctions between different degrees of response. The finding of a bimodal or trimodal population distribution of activity of a drug-metabolizing enzyme may indicate that the enzyme is coded by genes at a polymorphic locus.