For other uses, see Transition. Illustration of a transition: each of the 4 nucleotide changes between purines or between pyrimidines (in blue). The 8 other changes are transversions (in red).

Transition, in genetics and molecular biology, refers to a point mutation that changes a purine nucleotide to another purine (A G), or a pyrimidine nucleotide to another pyrimidine (C T). Approximately two out of three single nucleotide polymorphisms (SNPs) are transitions.[1]

Transitions can be caused by oxidative deamination and tautomerization.[2] Although there are twice as many possible transversions, transitions appear more often in genomes, possibly due to the molecular mechanisms that generate them.[3]

5-Methylcytosine is more prone to transition than unmethylated cytosine, due to spontaneous deamination. This mechanism is important because it dictates the rarity of CpG islands.

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Transition (genetics) - Wikipedia