title: Findings on the atopic triad from a Danish twin registry.
abstract: OBJECTIVE
  To estimate to what extent the same genetic and environmental risk factors influence asthma, hay fever and eczema.
  
  
  METHODS
  From the nationwide Danish Twin Registry, twin cohorts born between 1953 and 1982 were contacted for a questionnaire survey, and a total of 29 183 twin individuals (86%) responded. Subjects were classified as cases when responding affirmatively to three questions about the lifetime occurrence of asthma, hay fever and eczema. Variance components twin analysis was conducted using maximum likelihood methods.
  
  
  RESULTS
  The phenotypic (within-subject) correlations in liability between the different diseases were 0.57 (95% CI 0.54-0.59) for asthma and hay fever, 0.40 (95% CI 0.36-0.42) for asthma and eczema, and 0.33 (95% CI 0.29-0.36) for hay fever and eczema. Decomposition of these correlations into their genetic and environmental contributions showed that shared genes explained between 70% and 85% of the correlation between the different diseases. The remaining parts were explained by environmental factors shared between the diseases.
  
  
  CONCLUSIONS
  To a large extent, atopic diseases share a common genetic background, although disease-specific genes also play a considerable role. These results can prove informative when counselling families with atopy, and may furthermore be used to guide the search for pleiotropic genes of importance for these diseases.
authors.affiliation: Department of Respiratory Medicine, Bispebjerg Hospital, Copenhagen, Denmark. sft@city.dk
authors.forename: S F
authors.initials: SF
authors.lastname: Thomsen
authors.forename: C S
authors.initials: CS
authors.lastname: Ulrik
authors.forename: K O
authors.initials: KO
authors.lastname: Kyvik
authors.forename: L R
authors.initials: LR
authors.lastname: Skadhauge
authors.forename: I
authors.initials: I
authors.lastname: Steffensen
authors.forename: V
authors.initials: V
authors.lastname: Backer
mesh_terms: D000293:Adolescent; D000328:Adult; D001249:Asthma / Q000453:epidemiology*; D002648:Child; D003718:Denmark / Q000453:epidemiology; D003876:Dermatitis, Atopic / Q000453:epidemiology*; D004200:Diseases in Twins / Q000453:epidemiology*; D005260:Female; D006801:Humans; D008297:Male; D016017:Odds Ratio; D015995:Prevalence; D012042:Registries; D006255:Rhinitis, Allergic, Seasonal / Q000453:epidemiology*; D012307:Risk Factors; D011795:Surveys and Questionnaires