Strand-symmetric relative abundance functionals for di-, tri-, and 
tetranucleotides are introduced and applied to sequences encompassing a broad 
phylogenetic range to discern tendencies and anomalies in the occurrences of 
these short oligonucleotides within and between genomic sequences. For 
dinucleotides, TA is almost universally under-represented, with the exception of 
vertebrate mitochondrial genomes, and CG is strongly under-represented in 
vertebrates and in mitochondrial genomes. The traditional 
methylation/deamination/mutation hypothesis for the rarity of CG does not 
adequately account for the observed deficiencies in certain sequences, notably 
the mitochondrial genomes, yeast, and Neurospora crassa, which lack the standard 
CpG methylase. Homodinucleotides (AA.TT, CC.GG) and larger homooligonucleotides 
are over-represented in many organisms, perhaps due to polymerase slippage 
events. For trinucleotides, GCA.TGC tends to be under-represented in phage, 
human viral, and eukaryotic sequences, and CTA.TAG is strongly under-represented 
in many prokaryotic, eukaryotic, and viral sequences. The CCA.TGG triplet is 
ubiquitously over-represented in human viral and eukaryotic sequences. Among the 
tetranucleotides, several four-base-pair palindromes tend to be 
under-represented in phage sequences, probably as a means of restriction 
avoidance. The tetranucleotide CTAG is observed to be rare in virtually all 
bacterial genomes and some phage genomes. Explanations for these over- and 
under-representations in terms of DNA/RNA structures and regulatory mechanisms 
are considered.