The genetic fallacy occurs when an arguer evaluates whether a claim is good or bad, true or false based on the origin (or perceived origin) of the claim, rather than on the merits of the claim itself. It is a fallacy because claims can be good or bad irrespective of their origins.

Here is an example:

Dina: "Ben told me something about the big game. He says the Patriots are highly likely to win the Super Bowl." ​Juan: "He only says that because he's always been a totally obsessed Patriots fan. So there's no good reason to think they'll win."

Juan is overlooking that there may still be good reasons for believing the Patriots are likely to win, irrespective of Ben's opinion being rooted in his enthusiasm as a fan.

Here is another example:

Ross and Gina are on a trek in a remote forest.

Ross: "The natives told me these mushrooms will make you sick and possibly kill you."

Gina: "But they only believe it's deadly because a shaman filled their heads with superstitions. He's taught them that tiny invisible demons inside the mushrooms will cast spells on you to make you sick. We can ignore their irrational fears."

Ross: "Ummm.... I'm still not going to eat them."

Ross is probably wise. Just because the natives' aversion to the mushrooms is rooted in superstitious beliefs with dubious metaphysical assumptions, does not mean that their belief about the lethality of the mushrooms is incorrect.