Can anyone give me an example in simple DNA sequence variation when tajima D is positive

[hui]

According to the examples in this webpage ( http://en.wikipedia.org/wiki/Tajima’s_D ), it seems that pairwise difference between any two sequence pair never exceeds the number of segregating sites. How could their difference lead to a positive d, which results in positive tajima D? The example can look like this:

>sequence1: TTTTT

>sequence2: ATTGT

>sequence3: ATTGT

 

segregating sites: 2

average pairwise difference: (2+2+0)/3=4/3

 

Thanks a lot!