= Nucleotide transversion
A <nucleotide transversion> is a <nucleotide substitution> from a <purine>-derived base to a <pyrimidine>-derived base, or the reverse. In <DNA>, the unordered pairs are $A\leftrightarrow C$, $A\leftrightarrow T$, $G\leftrightarrow C$ and $G\leftrightarrow T$. A <nucleotide transition> instead stays within one of these chemical classes.<A simple method for estimating evolutionary rates of base substitutions through comparative studies of nucleotide sequences by Motoo Kimura>[Summary: transversion/type-II differences]{ref}
Each starting base has two possible transversion targets and one transition target. Thus twelve directed changes split into eight transversions and four transitions. Equal counts of possible events do not imply equal observed frequencies: chemical <mutation> mechanisms and <natural selection> can affect those frequencies.
Suppose an illustrative <sequence alignment> contains twelve transition differences and eight transversion differences. Its observed <transition-transversion ratio> is $12/8=1.5$. That ratio summarizes the scored differences; repeated substitutions at the same site can make it differ from the ratio of underlying instantaneous rates. The two-class distinction motivated the <Kimura two-parameter model>.<A simple method for estimating evolutionary rates of base substitutions through comparative studies of nucleotide sequences by Motoo Kimura>[Summary and evolutionary-distance formulation]{ref}
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