Morphology alone does not make an isochromosome.

Isochromosomes are chromosomes with genetically identical arms. Chromosomes morphologically similar to isochromosomes can arise from alternative mechanisms: whole-arm translocations and crossing over within inversion loops. Cases are presented which could have arisen by each of these latter two mechanisms. The first case is 46,XX,t(15;15) (qter-cen-qter;pter-cen-pter) and the second 46,XY,rec(18),dup q,inv(18) (p11.32q11.2).