Responding to the Common Genes Don't Imply Common Descent Canard

The next time someone tries an argument like this:

I'd like to ask how you can justify your leap from the existence of similarities and "common genes", even to a high degree of similarity, to "we have a common ancestor."... how can you know this Designer didn't make the DNA and RNA that way

Tell them the same way we do it in paternity cases. And suggest that they use that argument the next time they get involved in such a case. I'd love to see how a judge would react. Be sure you're there to witness the carnage.