Of course I can use the modulo operator (and I sure do).

I suppose you're right. It'll just look like

(x - 1) % size + 1
[download]

instead of

x % size
[download]

I don't think I pay any performance price

Well, there are more ops, but the real cost is the complexity and clarity due to working with off-by-one numbers (first element of genome in second element of array).