The best way to refuse is to just say "I'm good man" or something like that. Now if they keep trying to get you to do it then just leave...or do it[which i do not recommend considering most drugs are illegal when it comes to any use that isn't prescribed by a medical profesional such as your doctor or someone above him/her],but yeah it's best to just say no or leave.
They have half the chromosomes in body cells. One half are from one parent the other half is from the other parent.
