What do you think causes someone can be an extreme difference from their family?

We've all heard of the "black sheep" of the family. The "embarassment", etc.

But on the other hand there's people who are extraordinary examples of humanity while the rest of the family is selfish & self centered. My colleague is one of these. She is selfless, considerate of others, very friendly, etc. and yet her family is the almost complete opposite.

What do you think causes someone can be an extreme difference from their family?
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