Probability in-frameshift coupling

[Function]

Hi everyone

 

In preparation for my exam molecular biology, this question was asked:

 

When a cDNA-fragment of a eukaryotic gene is built in in a vector on a position after the lacZ translation-initiation place with EcoR1-linkers, the chance of getting an in-frame coupling would then be:

 

A. 1/2

B. 1/3

C. 1/6

D. Very small

 

And the answer would be C. However, I can't seem to understand why exactly. Could someone help me on this one?

I'd manage to find out already 1/3 (GAATTC can be built in in 3 ways, of which 1 won't cause a frameshift).

 

Thanks!

 

F.