Как то так?
BASES = c("A","C","G","T")
triN = apply(expand.grid(BASES,BASES,BASES),1,paste,collapse="")
data.frame(number=1:length(triN),codon=sort(triN))
number codon
1 1 AAA
2 2 AAC
3 3 AAG
4 4 AAT
5 5 ACA
6 6 ACC
7 7 ACG
8 8 ACT
9 9 AGA
10 10 AGC
11 11 AGG
12 12 AGT