The proposed mechanism of r(X) and i(Xq) formation. A: During the pachytene phase of meiosis I, a clinically silent inverted and duplicated segment X p11.1-p11.22 (green arrow) can form a loop with a normal X chromosome. Asymmetric crossing over within the loop may lead to the formation of dicentric chromosome with tandem duplication between the centromeres. B: After the zygote is formed, the dicentric chromosome could undergo fission with the formation of two cell lines. C: The cell retaining chromosome with terminal deletion and pericentromeric duplication of the short arm results in cell line 46,X,der(X)del(X)(p11.23)dup(X)(p11.21p11.22). D: The other cell retaining chromosome X with a complete deletion of the short arm results in cell line 46,X,r(X)(q11.1q13.1). E: Mitotically unstable del(Xp) could undergo stabilization through postzygotic terminal deletion of the long arm with self-circularization resulting in cell line r(X).