Which gene determines what letter is used to represent a pair of traits?

The dominant gene determines what letter is used to represent a pair of letters.

Explanation:

Example: If brown eyes are dominant (B) and blue eyes are recessive (b) then...

BB means: Brown

Bb: still means brown because there is still a dominant gene

bB: still means brown because there is still a dominant gene

bb: means blue, no dominant gene

Hope this helps!