The Genetic Fallacy is one in which an argument is supported or dismissed based on its origin, rather than its merit. It is similar to the Ad Hominem attack, and can even be easily confused with Ad Hominem, but rather than attacking the person making the argument, the source is attacked.
Here's an example to explain the difference:
Tim: I heard on the radio that France's Universal Healthcare System is misleading. Everyone has insurance through the government, but 90% of French people also have private insurance to supplement the shortcomings of their government system. What's the point of paying high taxes so everyone can have health insurance if that insurance doesn't cover what people actually need?
Susan: That was Glenn Beck who said that, you can't believe what he says because he's a right-wing conspiracy theorist nut-job.
This is an example of the Genetic Fallacy because Glenn Beck is the source, not Tim. Had Susan dismissed Tim as a right-wing conspiracy theorist nut-job, it would have been an Ad Hominem. Susan is committing the Genetic Fallacy because the nature of Glenn Beck's political views, belief in conspiracy theories, and state of mental health have nothing to do with whether or not French people need supplementary health insurance. Likewise, had Susan embraced Tim's argument based on having a celebrity-crush on Glenn Beck, she would have also been committing the Genetic Fallacy.
UPDATE: Fellow blogger Matthew Flannagan has been posting on logic and fallacies on his blog, and featured a post about The Genetic Fallacy. (HT: Apologetics 315 via Twitter)
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