Transition mutation

A Transition mutation occurs when a purine base (adenine/guanine) is substituted for a fellow purine base or a pyrimidine base (cytosine/thymine) is substituted for a fellow pyrimidine base. It is the opposite of a transversion mutation which occurs when a pyrimidine is exchanged for a purine base, or vice-versa.

References

Hartl D, Ruvolo M (2011), Genetics 8th edition, Massachusettes, Jones & Bartlett Learning