Genetic Fallacy

This is where the present claims are assumed to be true because of the origin of a person, group, or thing. That origin is however in most cases largely or completely irrelevant to the matter at hand and cannot be used as a basis for determining the truth of the present proposition.

This is an example of offering up irrelevant evidence or flimsy evidence to support a desired claim.

Examples

Dogs are essentially blood thirsty beasts, because they're descended from wolves after all.

The Johnsons are descended from people were always scamming our ancestors, so of course we can't trust them.

My parents always brought me up to believe that there is a god, therefore today I assert there is a god. [Self-origin.]

Weaknesses

The Genetic fallacy is basically a tactic to cast a present claim as true without going through the trouble of actual debate, analysis, or fact-finding, probably because the present claim is weak, unsupported, or simply bogus.

You should point out that the long-ago origin or characteristic has no bearing on the present discussion, and demand that they prove that it does. Extraordinary claims require extraordinary proofs...

You can draw attention to the likelihood that the long-ago proposition...

 *  May have been based on a lack of evidence.
 *  May have been the result of political spin at the time.
 *  May not have been conveyed to the present day truthfully.
 *  May have been the result of rumors and outright lies.
 *  May have been a white lie  meant to keep innocents out of trouble e.g. god-existence.
 *  Was not have been well reasoned anyway.