25Genetic Epidemiology
Genetic epidemiology is the study of the etiology, distribution, and control of
disease in groups of relatives and of inherited causes of disease in populations.
From its parent disciplines of genetics and epidemiology, it has inherited the key
elements of studying defi ned populations while investigating the roles of genes and
the environment in relation to each other and endeavoring to account for the known
biology of diseases. Quantifying the risk associated with genetic variation is a pre-
requisite for assessing the use of this new knowledge in medicine.
Research in disease etiology has shifted towards investigating genetic causes, pow-
ered by the human genome project. Successful identifi cation of genes for monogenic
disease has led to interest in investigating the genetic component of diseases that are
often termed complex that is, they are known to aggregate in families but do not segre-
gate in a mendelian fashion. Genetic epidemiology has permitted identifi cation of genes
affecting people’s susceptibility to disease. While the role of genetic factors in diseases
such as hypertension, asthma, and depression are being intensively studied, family stud-
ies and the large geographical and temporal variation in the occurrence of many dis-
eases indicate a major role of the environment. Thus, it is necessary to consider fi ndings
about susceptibility genes in the context of a population and evaluate the role of genetic
factors in relation to other etiological factors. Several approaches have been used to
resolve the genetics of disease and to study the relation of genes to environmental fac-
tors in the population. Until now, population screening involving genetics has focused
on the identifi cation of persons with certain mendelian disorders before the appearance
of symptoms and thus on the prevention of illness. In the future, we are likely to screen
entire populations or specifi c subgroups for genetic information in order to target inter-
vention in individual patients for the purpose of prevention of disease.
Limitations of Medical Genetics and Future Prospects
Some of the limitations of investigations into the genetic basis of disease are:
- Shortage of medical geneticists.
- Disease phenotypes have been under-appreciated by geneticists. Ideally investi-
gators should initially study phenotypes without knowing genotypes to ensure
that the latter does not unduly infl uence the analysis of the former. - Extended pedigrees of affected families have not been studied adequately.
- Genetic linkage studies often have different, even confl icting results. There is
need for multiple groups to collaborate and pool their data to discover the part of
the genetic “signal” on which they can agree. - Statistical methods for study of medical genetics need to be greatly improved.
- Genetic variants involved in common diseases are of low to moderate penetrance,
i.e., only some carriers will develop the disease. Many of these moderately-
penetrant gene variants may be diffi cult to detect using classical methods of
genetic research. New methods need to be specifi cally designed to identify these
types of gene variants. This information can be used to improve healthcare
through disease risk-reduction, earlier diagnosis and more specifi c therapies.
Role of Genetics in Future Approaches to Healthcare