My oldest daughter is turning 6 soon, and she is already showing signs that both my wife and I have noticed. Extreme mood swings, extraordinary tantrums, intense energy. And I don't mean "normal" tantrums and energy for her age. Even her friends and other parents noticed her intensity.
I don't mind at all. I love her no matter what she might have, she takes after her old dad.
My youngest daughter is the opposite, very mellow and overall happy kid. She is more like my wife.

Obviously it is too early to tell anything, but my oldest daughter's personality is eerily similar to my own. Bipolar runs strong in my family. My dad is type 1, sister is type 2.