Principles of polar body biopsy

Schematic diagram of meiosis I and II, illustrating different outcomes for the primary oocyte and polar body I and secondary oocyte and polar body II, when non-crossing over and crossing over occur. In the absence of crossing over, analysis of the polar body I enables one to deduce the genetic status of the oocyte and, hence, whether the zygote could be affected. If crossing over has occurred between homologous chromosomes, polar body I analysis alone will be noninformative, whereas polar body I and polar body II analysis enables one to deduce the actual maternal contribution to the zygote and a normal or abnormal chromosome constitution.
(A) Meiosis I, no crossing over, normal primary oocyte and affected polar body I.
(B) Meiosis I, no crossing over, affected primary oocyte and normal polar body I.
(C) Meiosis I, crossing over, polar body I is uninformative.
(D) Meiosis II, crossing over, polar body II is informative, secondary oocyte affected.
(E) Meiosis II, crossing over, polar body II is informative, secondary oocyte unaffected.