My answer to this Quora question was originally published on 10/16/2013.
Yes, you should tell your child. Because he or she WILL definitely one day find out. Read the other answers for insight into how devastating this can be.
But let's say you want to go against Quoran advice. Maybe you think you are pretty crafty and can manage to hide such a big secret for your child's entire life, even after you are gone.
Well, maybe in past centuries people were able to hide such things, but with the way technology is evolving, I can't see how you'd be able to hide it.
Remember, your child will probably live 80+ years. What will medicine be like in the year 2050? 2070? 2090? I imagine we will rely heavily on genetic analysis, because in 2013, you can already get your DNA analyzed for $300. So one day your child is certain to discover that his/her genetic history does not match your family folklore. And then your secret is out. Good luck explaining the lies to them.