Third letters in codons counterbalance the (G + C)‐content of their first and second letters

The correlation among the frequencies of appearance of G and C bases at the first, second and third positions of codons is examined. It is found that the redundancy at the wobbling third base is utilized to counter‐balance the local (G + C)‐content variation due to the first and second bases of codons, so that a homogeneous (G + C)‐content is provided to each individual gene. We speculate that this levelling tendency of (G + C)‐content comes from the functional constraint to achieve a uniform double‐helix stability in a gene.