ABSTRACT
Association studies may be defined as population-based genetic studies that
examine whether a particular allele or a series of alleles in one or more
genes occur at a significantly higher rate with a disease phenotype than
would otherwise be expected. This method yields statistical estimates of whether a polymorphism is a marker, or contributor, to disease susceptibil-
ity. A positive association may occur because: (i) the polymorphism is a
true causal variant, (ii) of a type I error (an association was detected that
occurred by chance), (iii) of inherent study biases, or (iv) of linkage with
a causal variant. The inability to detect an association may be because of:
(i) the polymorphism is not causal, (ii) type II error (not detecting an asso-
ciation when one exists) usually because of a lack of statistical power, or
(iii) lack or weakness of a necessary environmental stimulus (1). Despite the high potential for erroneously identifying a polymorphism as a causal
variant, association studies to date have been highly successful in identify-
ing genes involved in the development of asthma. This is likely to be
because of the large number of association studies completed due to their relatively low cost and the process by which a candidate gene is selected.
