Figure 3A: Mechanisms for isochromosome formation: Breakage and reunion during first meiotic prophase resulting in formation of a dicentric isochromosome which was retained in the gamete (defective gametogenesis) [34]. Subsequent fertilization with defective sperm gamete leading to abnormal embryogenesis B: Isochromosome formation due to errors in meiotic non disjunction and trisomy rescue mechanism at embryogenesis.