The genetic fallacy is not a flaw in one's DNA, but rather an invalid form of argumentation. It involves contending that an argument is false because of some negative characteristic attributed to the person, publication, or group advancing the argument. Philosophically, it is flawed because it doesn't address the substance of the argument it purports to rebut. Psychologically, it may be attributable to ignorance of correct argumentation, or to intellectual laziness (one doesn't want to take the trouble to deal with an argument properly), or to intellectual dishonesty (the person committing the fallacy understands perfectly well that they aren't presenting a cogent argument, but they realize that they can sway careless, ignorant readers and listeners through an appeal to non-rational impulses).
An AP provided an excellent example of the genetic fallacy in his/her reply to a response of mine in the AlterNet Movie Mix thread (see also my subsequent response). in another thread, slashgirl seems to have been concerned that her arguments concerning the truth of Christianity would be dismissed because of her beliefs when she said "But then what do I know, I'm only a pagan." I suspect she has good reason to fear that, because she has doubtless encountered Christians who have committed the genetic fallacy against her in precisely this way. While I'm persuaded that her objections could be more than amply met, it would be unthinkable for me to dismiss them simply because she is a pagan. She is entitled to answers that address the questions she has raised.
One substantial factor contributing to the polarization of today's political discourse is the shoddy reasoning (or unreasoning?) engaged in by so many on all sides, and the genetic fallacy has become a prince among fallacies, because it feeds our demonization of "the other side". And I'll be the first to concede that the right is as bad as the left on this score. The best that can be said for those who know better is that they are intellectually lazy. An inspired man once gave this instruction to those whom he was leading: "Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect". Note the emphasis on reason, gentleness, and respect. I trust that even those who don't agree with me that this man was inspired can see the merit in what he wrote.
One may, I think, consider the source of a fact that is presented in forming a tentative judgement about its likely correctness. A person may feel they have sufficient reason to doubt the accuracy of the NYT, The National Review, The Nation, Rush Limbaugh, Michael Moore, etc. We don't always have the time or the means to chase down every important fact presented to us, and as long as we regard our judgement about the likely correctness of the fact as tentative, and as long as we are open to correction if we are shown to be wrong, I think we are within our rights in "considering the source". However, I think we are never within our rights in dismissing the argument of another simply because of who the other is who is advancing it. We may be silent; we may say "I need to think about this before I form an opinion about the correctness of this argument"; we may offer a tentative judgement about the argument in the form of an opinion; we may offer misgivings about those parts of the argument that are clear to us. But it is always wrong to dismiss an argument conclusively on any basis other than a reasoned consideration of its substance. We owe one another nothing less than to engage one another's arguments substantively. I hope I have always done this on LISNews. If I haven't, I welcome correction.