  In this paper we present geometric features of group based models. We focus
on the 3-Kimura model. We present a precise geometric description of the
variety associated to any tree on a Zariski open set. In particular this set
contains all biologically meaningful points. Our motivation is a conjecture of
Sturmfels and Sullivant on the degree in which the ideal associated to 3-Kimura
model is generated.
