How is Frankenstein an example of science fiction?

Shelley's work can be considered part of the science
fiction because it ponders the fundamental question of the genre in "What if..." 
Science fiction is primarily defined by thinkers posing this question in different
contexts and conditions.  What Shelley has rendered is just that in her question of
"What if one could clone life?"  Victor's exploits in science help him to discover a
world that is different than the one he traditionally knows of and in which he lives. 
This is an aspect of science fiction in that it takes the idea of what could be and
manifests it into the possible explanations.  Another reason why the work could be
considered science fiction is because it shows that the result of this exploration is a
disastrous one.  The construction of life is one that turns out to possess terrible ends
for him and this is where the science fiction ends are most present.  The genre of
science fiction is one where what might be seen as good and optimistic is one revealed
to be far worse.  In this light, Shelley's work can be seen as science
fiction.