Association: 1. In dysmorphology (the study of birth defects), the nonrandom occurrence in two or more individuals of a pattern of multiple anomalies (birth defects) not known to be a malformation syndrome (such as Down syndrome), a malformation sequence (of events) or what is called a polytopic field defect (in which all of the defects are concentrated in one particular area of the body). An example of an association in dysmorphology is the VACTERL association of birth defects.

2. In genetics, the occurrence together of two or more characteristics more often than would be expected by chance alone. An example of association involves a feature on the surface of white blood cells called HLA (HLA stands for human leukocyte antigen). A particular HLA type, HLA type B-27, is associated with an increased risk for a number of diseases including ankylosing spondylitis. The extent of the association is enormous. Ankylosing spondylitis is 87 times more likely to occur in people with HLA B-27 than in the general population.