Crohn's disease and ulcerative colitis are related, complex genetic disorders. A present hypothesis holds that several genes are associated with inflammatory bowel disease, some of which are common to both Crohn's disease and ulcerative colitis and others associated exclusively with one disease or the other. In complex genetic diseases, gene-gene and gene-environment interactions affect the final phenotypic expression. Therefore, careful refinement of phenotypic classifications through clinical examination and subclinical markers may assist in the search for genes for specific diseases. Given the association of inflammatory bowel disease with other autoimmune diseases (eg, psoriasis, ankylosing spondylitis), the possibility of common 'autoimmune genes' is discussed. Progress in candidate genes is reviewed. Genome-wide searches in families multiply affected with inflammatory bowel disease have demonstrated a confirmed linkage region for Crohn's disease on chromosome 16 and evidence for linkage on chromosome 12 in families with inflammatory bowel disease.
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