Ab initio models of GexAs10Se90-x, and GexSb10Se90-x glasses are constructed, and their bonding environments are characterized and compared against each other and to recent experimental studies of equivalent glasses at the same stoichiometry and density. A minimum in the linear refractive index is found to correlate with a maximum in the number of length-one, predominantly Se, atomic chains for both glass types. The threshold behavior difference between GeAsSe and GeSbSe is shown to be due to the appearance of As-As-Se-2 structural units beyond the MCN = 2.67 threshold in the GeAsSe glasses.