Because it’s so trivial[1] that Bill’s inability to do it himself is further evidence that he hasn’t passed high-school maths let alone university level courses?
Of course it has absolutely nothing to do with genetics, since genetic sequences and proteins aren’t fixed-length, among other things.
number of possible sequences of l elements each of which has n possibilities is (n×n×n×…×n) = n^l. No probability required. ↩︎