(*** Spoilerama ***)
=v= _Futurama_ is bad for the economy. Just think of all the otherwise-productive people who've spent the day puzzling out genetic quandries.
=v= We must first realize that it may all be a red herring and Mildred was already pregnant with Enos' baby. If for no other reason, this is suggested by Enos' physical resemblance to Fry (and to the young Professor Farnsworth, as well as Cubert).
=v= If not, then consider: Fry's father would normally be the result of half of Fry's DNA strand, plus half of Mildred's. And when he conceived Fry, the DNA strand would normally be only about 1/4th of Fry's. So, Fry's Mom is genetically the anti-Mildred, and/or the F-ray must have strengthened Fry's DNA to magical elf proportions, and/or atomic testing weakened Mildred's DNA, and/or a Nibblonian wizard did it.
=v= Also, if Fry is Farnsworth (great * x) uncle, then he's also his (great * (x + 1)) grandfather. I wonder why neither Leela's nor Farnsworth's machines figured that out. <_Jym_>