ABSTRACT
It is well recognized that some human diseases such as cystic fibrosis run in
families and that their inheritance follows a Mendelian pattern explained by the
presence of a single aberrant gene. Other conditions occur in relatives of affected
individuals at a higher rate than in the general population and this most likely
reflects the interaction of one or more genes with one or more environmental
factors. Such disorders are defined genetically as complex diseases. They include
a number of common disorders such as hypertension, type II diabetes, and some of
the diffuse lung diseases, especially systemic sclerosis and sarcoidosis. The
strength of the genetic component of a particular disease can be assessed using
genetic epidemiological techniques such as twin studies or family studies. The
genetic component of disease susceptibility can be assessed by estimating the
relative risk, defined as the risk of disease in the relative of an affected individual
related to the risk in the general population. Such relative risk is defined as
l with ls being the relative risk for a sibling and lr being the risk for a relative, usually first degree relatives. In general, the higher the relative risk, the more
likely there will be a genetic component and thus the likelihood of identifying
that component(s). Nonetheless, analysis of complex diseases raises particular
difficulties and requires specific methodologies (1).
