Association (genetics)

In genetics, the word "association" has a very specific and special meaning. It refers to the occurrence together of two or more characteristics more often than would be expected by chance alone. Association is to be distinguished from linkage: the tendency for certain genes to be inherited together because they are on the same chromosome. Let us take linkage first. The genes for red-green colorblindness, hemophilia and Duchenne muscular dystrophy might be found to be linked, since all three genes are on the same chromosome, the X chromosome, and they are passed along as a package to a child. An example of association involves blood types and duodenal ulcer. A person with blood type O has almost a 60% greater risk of a duodenal ulcer than someone with blood type AB (which is coded at the same chromosome location as blood group O). Blood group O and duodenal ulcer are said to be associated. An even stronger association involves a feature on the surface of white blood cells, the HLA type (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. The disease is 87 times more likely in bearers of HLA B-27 than in the rest of the population.