Possible mechanisms for the formation of neocentric marker chromosome in the patient. Chromatids breakage (red scissors) and a distal U-type exchange happened before sister chromatid separation during the meiosis II. Neocentromerization occurred once breakage happened. Deep color ovals are the centromeres, and the same one on the acentric fragment is neocentromere. The broken ends (red wave) of the centric fragment could be stabilized by telomere restitution. After zygogenesis and subsequent replication the result would be partial trisomy for the duplicated fragment.