Down syndrome (DS) is caused by trisomy for human chromosome 21. Individuals with DS commonly exhibit mental retardation, which is associated with abnormal brain development. In the neocortex of the DS brain, the density of neurons is markedly reduced, whereas that of astrocytes is increased. Similar to abnormalities seen in DS brains, mouse models of DS show deficits in brain development, and neural progenitor cells that give rise to neurons and glia show dysregulation in their differentiation. These suggest that the dysregulation of progenitor fate choices contributes to alterations in the numbers of neurons and astrocytes in the DS brain. Nevertheless, the molecular basis underlying these defects remains largely unknown. We showed that the overexpression of two human chromosome 21 genes, DYRK1A and DSCR1, contributes to suppressed neuronal differentiation of progenitors in the Ts1Cje mouse model of DS. In addition, the effect of DYRK1A and DSCR1 overexpression on neuronal differentiation is mediated by excessive attenuation of the transcription factor NFATc. Additionally, we demonstrated that an increased dosage of DYRK1A contributes to elevated potential of Ts1Cje progenitors to differentiate into astrocytes and enhanced astrogliogenesis in the Ts1Cje neocortex. Further, we linked the increased dosage of DYRK1A to dysregulation of STAT, a transcription factor critical for astrogliogenesis. Together, our studies identify critical pathways responsible for the proper differentiation of neural progenitors into neurons and astrocytes, with direct implications for the anomalies in brain development observed in DS.