I believe that genius is essentially a combination of high intelligence, and high creativity (in the sense of a semi-psychotic, trance-like ability to think poetically and with loose-associations) - plus sufficient application to work very hard on those subjects which really inspire the genius.
A genius also needs an awkward personality, or else he will simply fit-in with the normal social expectations; and sufficient autonomy that this misfit status does not bother him.
(Note: these ideas mostly derive from HJ Eysenck's book - Genius.)
According to most studies, it looks as if most geniuses were of *very* high intelligence (general intelligence, 'g' or IQ) - being something like three or more standard deviations above average (IQ 145 plus when the average IQ is defined as 100 and the standard deviation as 15).
However, in my reading of the biographies of geniuses, some seem to be more more normal in intelligence than this - certainly in the top ten percent, say IQ 120 plus, but not more.
Just for fun, I will nominate Ludwig Wittgenstein as one of these.
To my mind, Wittgenstein seems to have a much lower IQ than most philosophers, he approaches things in a relatively straightforward manner. What is unusual about Wittgenstein is not his ability to think abstractly, reason extensively and learn rapidly - but his amazing persistence at picking away, with searing intensity and poetic expressivity, year after year, again and again, at matters which most people would regard as minutiae.
For what it is worth, Wittgenstein's academic record was very good but not amazing - which is at least consistent with the above.
Any other suggestions of only-moderately-high IQ-geniuses?
[I am assuming that Wittgenstein really was a genius, since he was so massively influential in 20th century philosophy - and was rated so highly by extremely intelligent people such as Bertrand Russell and Elizabeth Anscombe. However, Wittgenstein seems to me always to have been wrong about everything - and indeed twentieth century philosophy is always wrong about everything; so maybe Wittgenstein was not a genius but a nutter who happened to be taken seriously by a silly and corrupt area of intellectual endeavor, due to other aspects of his personality which led to a cult growing around him.]