I am interested in researching two very different problems (Genetics + Computer Vision). Haven't started a PhD yet
Your approach sounds reasonable to me.
Both problems do share some elements in common:


both operate on a larger data set, so efficient algorithms are needed.
both may profit from novel approaches to identify patterns


Both could be based on AI. I know greedy algorithms from traditional computing for genetic research as well as for vision. Why not trying that way with you specific ideas? 
As I did a PhD in high performance computing, I can recommend doing your way. You should have passion for your path; it may take a while.