Despite recent advances in our understanding of the molecular basis of skeletal fragility, little is known about how these molecular alterations lead to whole bone brittleness. In the current study, we investigated the relationship between a type-I collagen mutation and post-yield behavior of whole bone in Mov13 transgenic mice by considering tissue-level organizational issues known to be important for normal bone fracture. Mechanical assays revealed that the post-yield deflection of Mov13 femurs was reduced by 61%, relative to littermate controls. Fractographic images revealed that lamellar interfaces, which were important for dissipating energy during the failure process of control lemurs, were not effective in Mov13 mice. Further investigation revealed that a 22% reduction in bone collagen content, a 2-fold increase in tissue porosity, and significant alterations in collagen organization interfered with normal energy dissipation mechanisms of Mov13 microstructure. Collectively, the results provided the first evidence that the reduced ductility associated with a type-I collagen mutation was mediated by alterations in intermediate structures that normally contribute to the post-yield behavior of cortical bone. Thee results suggest that, to better understand the pathogenesis of skeletal fragility, it is important to consider the effects of molecular alterations on higher-level structures, particularly those structures that contribute to the failure mechanisms in normal bone.