Complex sentence: Misdivision of the centromere and U-type exchange can occur to sister chromatids, thus creating an isochromosome with genetically identical arms, but U-type exchange can also occur for homologous chromosomes which creates an isochromosome with homologous arms.
Split into simpler sentences: Misdivision of the centromere and U-type exchange can occur in sister chromatids, thus creating an isochromosome with genetically identical arms. However, U-type exchange can also occur for homologous chromosomes which creates an isochromosome with homologous arms.