I did Genetics & Breeding as one of my courses at college (and I got a distinction, but come on, it's basically a course all about sex!) so...
It depends whether the gene for power or no-power is dominant.
I may now waffle: I did a thing about rex rats at college, rex rats are the ones with curly hair, but if you breed two rexes together, you end up with a bald rat.
So, a rex rat has the genes Rr. A normal haired rat has the genes rr. And a bald rat has the genes RR. (Capital letters are dominant).
A rex breeds with a rex and the dominant R takes over so you get a baldy RR. Two normal haired rats breed and you get a normal rat rr. One normal rat and one rex rat breed and you get another rex: Rr.
Um. So, maybe you could apply that to your powers - give them genetic letters, work out which is dominant and which not. PP or something (so instead of a bald rat, you end up with a person more powerful than their parent if you wanted).
This is all very true for Mendelian inheritance http://en.wikipedia.org/wiki/Mendelian_inheritance
i.e. traits for which the genes are known and simply classified (e.g. for cystic fibrosis -- either you have it or you don't). But most traits aren't Mendelian, such as height -- there aren't three different heights you can have (tall, middle and small) but an infinite number of heights. Genes which govern traits such as height are called (I think, because I haven't been able to verify this) genes of minor effect.
What does this mean for springs? If you don't delve too deep trying to explain the genetics of it, your trait could be the same, more, or less in the child. As TJ said, the "normal" parent may have some unexpressed genes for the trait, which combine with the other's many expressed genes to make the child stronger.