Single layer SiGe/Si and Si/SiGe heterostructures were grown by the molecular beam epitaxy technique with Si solid and GeH4 gas sources. Dependence of dislocation distribution in heterostructures along the growth direction on alloy composition and layer thickness was studied using transmission electron microscopy and chemical etching/Nomarski microscopy. It has been found that misfit dislocation network evolves from hat regular to dense three-dimensional during the epitaxial growth. This process is strongly affected by alloy composition and accompanied by complex changing in dislocation density in both the layer and substrate. The phenomenon is interpreted qualitatively in terms of Ge-rich microsegregation effect on dislocation generation, propagation and multiplication.