In a mutational event, when adenine is replaced by guanine, it is a case of

Transition are substitution gene mutation in which a purine (adenine) is replaced by another purine (guanine) or a pyrimidine (thymine is replaced by another pyrimidine cytosine). Change of codon ATC to GTC or ATT or ACC is an example of transition. Transversion in substitution gene mutation in which a purine $(\mathrm{A}$ or G$)$ is replaced by a pyrimidine (T or C) or vice-versa.