A transversion is a DNA point mutation that interchanges a two-ring purine nucleobase with a one-ring pyrimidine, or vice-versa; conversely, a transition substitutes one nucleobase for another having the same ring structure (see the figure below). Transitions and transversions can be defined analogously in terms of RNA.
Because they do not change the structure of the mutated nucleobase, transitions occur about twice as often as transversions. Transitions appear even more often in coding regions because they are less likely to alter the underlying amino acids that the mutated base codes for, especially if they appear as the third nucleotide of a codon. As a result, we can use the ratio of transitions to transversions as a test for the presence of coding regions when comparing two genomes.