Genetic susceptibility is the genetic load able to influence in the phenotype of a person, population or species. Phenotype is known as the expression of the genotype (from DNA) depending on the environment. It’s the visible manifestation of the genotype and it refers to any characteristic or trait of a person; it involves morphology, physiology, behavior, biochemical properties, etc. Therefore, genetic susceptibility is the increase or decrease on the probability of contracting an illness depending on genetics. Through simple nucleotide polymorphisms (SNPs) and following a multiplicative mathematical model with exceptions as haplotypes, it is possible to calculate the risk of developing a disease that a particular person could have, through incidence and allele frequencies.
It is the biological field that studies the effect of individuals' genetic variability in their response to certain drugs. It offers the possibility of developing a new generation of drugs, optimizing their effectiveness and safety and therefore having important implications in clinical practice. A better understanding of pharmacogenetics will facilitate personalized medicine, tailoring therapy to each patient. We would be able to identify those drugs that are more effective, the most appropriate dose and the optimum concentrations of them in blood, avoiding undesirable side effects. This new tendency will allow moving from empiricism when prescribing drugs, reducing substantially the necessity for hospitalization and its associated costs. When a drug is administered, it is absorbed and distributed to its point of action where it interacts with its substrate (receptors and enzymes); it is metabolized and then excreted. In each of these steps, there may exist a genetic variation with a different clinical outcome. These inherited variations play a role in the carriage and distribution of the different drugs, as well as in their interaction with the therapeutical targets. The aim is to decode the growing complexity of the human genome and to obtain the necessary knowledge to understand how individual genetic variations influence drug metabolism. It remains a challenge for the upcoming decades.