What surprised me is when I realized that it was entirely possible (though maybe not probable) for siblings to share 0% of their DNA. That is, if one sibling received the "first" 50% of the father and mother's genes, and the other sibling received the "second" 50%, they would have 0 overlapping genes.
Wouldn't some of the genes be identical, ie you randomly get one of two identical copies? I'm thinking some of the essential genetic code sections might be of a nature where having them damaged would kill the kid.
That isn't possible. You got your Y from your dad if you are male. You got an X from your mom. So all siblings share mom's X and all sons share dad's Y. You also got your mitochondrial DNA from mom.
