How were mutants created in x-men first cl? Why did magneto say this?

I thought mutants in x-men were created by the x-gene which some people were born with, all thru the movie this seemed to apply, but somewhere between the middle and the end of the movie magneto was talking about mutants and he said "radiation gave birth to us" this confused me and my summer camp partner and we have to do a movie review together and we both agree we should state how mutants became mutants but because of this small complication we both have seperate views and we dont know what is right. Please help