Ti/Tv

DNA substitution mutations are of two types. Transitions are interchanges of two-ring purines (A G) or of one-ring pyrimidines (C T): they therefore involve bases of similar shape. Transversions are interchanges of purine for pyrimidine bases, which therefore involve exchange of one-ring and two-ring structures.

    Although there are twice as many possible transversions, because of the molecular mechanisms by which they are generated, transition mutations are generated at higher frequency  than transversions. As well, transitions are less likely to result in amino acid substitutions (due to "wobble"), and are therefore more likely to persist as "silent substitutions" in populations as single nucleotide polymorphisms (SNPs).

http://www.mun.ca/biology/scarr/Transitions_vs_Transversions.html

---

Transition to Transversion Ratio 

Human mutations don't occur randomly. In fact, transitions (changes from A <-> G and C <-> T) are expected to occur twice as frequently as transversions (changes from A <-> C, A <-> T, G <-> C or G <-> T). Thus, another useful diagnostic is the ratio of transitions to transversions in a particular set of SNP calls. This ratio is often evaluated separately for previously discovered and novel SNPs.

Across the entire genome the ratio of transitions to transversions is typically around 2. In protein coding regions, this ratio is typically higher, often a little above 3. The higher ratio occurs because, especially when they occur in the third base of a codon, transversions are much more likely to change the encoded amino acid. A refinement to this analysis, in protein coding regions, is to examine the transition to transversion ratio separately for non-degenerate, two-fold degenerate, three-fold degenerate and four-fold degenerate sites.

http://genome.sph.umich.edu/wiki/SNP_Call_Set_Properties

---

Some useful papers:

Transition-Transversion Bias Is Not Universal: A Counter Example from Grasshopper Pseudogenes 

http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.0030022

Estimation of the transition/transversion rate bias and species sampling

http://www.ncbi.nlm.nih.gov/pubmed/10093216

Mutational and fitness landscapes of an RNA virus revealed through population sequencing

http://www.nature.com/nature/journal/v505/n7485/full/nature12861.html