Allergic diseases are complex and involve a range of environmental
factors interacting with a susceptible genotype. The familial clustering
of diseases, such as asthma and hay fever, has been recognised for over
two centuries, but identification of the genetic basis to this had to
await the molecular biological revolution. Estimates of the contribution
that genetic factors make to asthma susceptibility range from 35% to
70%. For the majority of allergic diseases, segregation analysis has not
identified a consistent Mendelian pattern of inheritance, which, when
combined with multiple phenotypes and environmental interactions, has
made identifying candidate genes especially difficult and, at times,
controversial. Part of the difficulty has been lack of agreement over
phenotype definitions, reduced power of studies to predict linkage and
association, and, importantly, lack of true heterogeneity between
populations. Despite these difficulties, the last decade has witnessed
enormous progress in this field.