Early genetic research focused on identifying single genes responsible for specific familial disorders. However, radical technological advancements such as high throughput testing and genome-wide scanning techniques have made it possible to examine complex conditions influenced by multiple genes and environmental factors to determine population susceptibility. Genetic epidemiology studies the distribution of genetic traits and variation within families and populations, risk factors associated with the frequency of genetic traits, and the role of genetic factors in disease aetiology (Khoury, Beaty & Cohen, 19931). This enables the impact of a specific genetic variation on disease risk in an individual or in a population to be estimated (Kaprio, 20002). As the contribution of a single gene variant to disease can be relatively small, it is important to understand not only the contribution of other genetic factors but how these interact with environmental factors to modulate disease risk.