With degeneracy (different codons that code for the same amino acid) of the genetic code it is possible to have different base sequences in a gene and yet code for the same sequence of amino acids. In coding for leucine, e.g., the RNA codons for leucine are CUU and CUA which correspond to 33.3% (G + C), while its other two RNA codons are CUC and CUG and these are 66.7% (G + C). If the first codon only were used to code for leucine in a gene, and all other amino acids were coded with a similar bias in G + C, then the gene would have a G:C/A:T ratio of 1 : 2. This extreme situation never actually arises in nature because not all amino acids have four different codons.
