A new study of identical twins confirms that genetics are a poor, if not purposeless, prognostic of the chance of getting a disease.

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Are diseases genetic? That's the simplified and distorted mantra we hear every day in the media -- that scientists have just discovered the gene causing this or that disease.

The truth is that genes only very rarely cause diseases. An illuminating new study in the journal Science Translational Medicine helps clarify what geneticists have been trying to explain to us for years: genes influence, but they don't determine.

The just-published study examines how often identical twins get the same diseases. Reviewing records of 53,666 identical twins in the United States, Sweden, Finland, Denmark, and Norway, researchers tabulated how well genes predict the chance of getting a disease. The answer is that they really can't. Predictions based on genes turned out to be very close to useless. As Gina Kolata summed up in The New York Times: "While sequencing the entire DNA of individuals is proving fantastically useful in understanding diseases and finding new treatments, it is not a method that will, for the most part, predict a person's medical future."