  The usage frequencies for codons belonging to quartets are analized, over the
whole exonic region, for 92 biological species. Correlation is put into
evidence, between the usage frequencies of synonymous codons with third
nucleotide A and C and between the usage frequencies of non synonymous codons,
belonging to suitable subsets of the quartets, with the same third nucleotide.
A correlation is pointed out between amino acids belonging to subsets of the
set encoded by quartets of codons. It is remarked that the computed Shannon
entropy for quartets is weakly dependent on the biological species. The
observed correlations well fit in the mathematical scheme of the crystal basis
model of the genetic code.
