# Infer distance since duplication from tree?

Question
I have a really simple question, but I'm not able to figure it out.
I made a tree (using BLOSUM62). Here is a part of the tree:

How can I calculate the distance till the duplication event occured between HBD en HBB?

• just for HBB 36.21 and for HBD 38.79
• or 38.79 + 36.21 = 75

I determined if this set of sequences follows a molecular clock, and indeed this was the case. So I determined the rate at which these sequences envolve. If I can determine the distance between HBD and HBB I can devide this by the rate to obtain the time since the duplication event happened.

How could you calculate this one?

Assuming a molecular clock, the distance between the sequences and the duplication event is 75/2 = 37.5. Ideally, these sequences should have the same distance from the internal node (under a clock), and you could use a model that explicitly assumes a clock -- which will lead to ultrametric trees. One example is the program promlk (or dnamlk) from the phylip package. A faster alternative would be the UPGMA method, which also gives you an ultrametric tree.