Callosal dysgenesis (CD) is observed in many neurodevelopmental conditions, but its subjacent mechanisms are unknown, despite extensive research on animals. Here we employ magnetic resonance diffusion tensor imaging and tractography in human CD to reveal the aberrant circuitry of these brains. We searched particularly for evidence of plasticity. Four main findings are described--1) in the presence of a callosal remnant or a hypoplastic corpus callosum (CC), fibers therein largely connect the expected neocortical regions; 2) callosal remnants and hypoplastic CCs display a fiber topography similar to normal; 3) at least 2 long abnormal tracts are formed in patients with defective CC: the well-known Probst bundle (PB) and a so far unknown sigmoid, asymmetrical aberrant bundle connecting the frontal lobe with the contralateral occipitoparietal cortex; and 4) whereas the PB is topographically organized and has an ipsilateral U-connectivity, the sigmoid bundle is a long, heterotopic commissural tract. These observations suggest that when the developing human brain is confronted with factors that hamper CC fibers to cross the midline, some properties of the miswired fibers are maintained (such as side-by-side topography), whereas others are dramatically changed, leading to the formation of grossly abnormal white matter tracts.