Two genes for the same trait are compared. which one is not an example of a mutation? in the first, the series is adenine followed by two thymine; in the second, adenine is followed by two thymine. in the first, the series is cytosine - adenine - thymine; in the second, it is cytosine - adenine - adenine. the first has thymine where the second has a guanine. the first has a series of three nucleotide bases; the other has a series of four.