I would make this an instance method. But that might be due to the fact that I have no clue of genes ;)
Instance methods can be overridden by subclasses which greatly reduces the complexity of your code (less need for if-statements). In the static method example, what will happen I you get a specific type of gene for which the distance is calculated differently? Ad another static method? If you'd have to process a polymorphic list of genes you'd have to look a the type of gene to select the correct distance method... which increases coupling and complexity.
