The density and origin of dislocations in a variety of molecular-beam-epitaxy-(MBE)-grown Ge1-xSnx/Ge/Si (001) heterostructures, with Sn-content varying from 0.4 to 5.2%, have been investigated systematically by high resolution X-ray diffraction (HRXRD). In particular, using the approach due to Kaganer et al. (V.M. Kaganer et al., Phys. Rev. B 72, 045423 (2005)) for the first time to the Ge1-xSnx alloy, it is demonstrated that reliable estimates of both edge and screw dislocation densities can be obtained from HRXRD data. Based on the correlations of strain relaxation and dislocation densities of the alloy epilayers and the underlying Ge buffer layers, we observe that dislocations threading from the latter predominantly contribute to the strain relaxation of the former. Thus, Ge1-xSnx epilayers of sub-critical thicknesses can be made to relax significantly by growing them on partially-relaxed, relatively-thin Ge buffer layers. This may be promising for the realization of Ge1-xSnx epilayers with direct electronic band gap. (C) 2017 Elsevier B.V. All rights reserved.