Fig. 1. Segregation of X chromosome homologs and univalents at MI. Left: Cartoons depict MI segregation of (A) X homologs to the oocyte and polar body in control XX females, (B) intact segregation and (C) precocious segregation of sister chromatids (PSCS) of the univalent X chromosome in oocytes from XO females (Note that the termed ‘precocious’ refers to the fact that sister segregation occurs at MI rather than MII; a physical association is maintained between the sister chromatids until anaphase I). Right: Actual images of oocyte and polar body chromosomes in MII arrested oocytes from XX and XO females, hybridized with an X-chromosome specific probe. In all images, the group of chromosomes (red) segregated to the polar body at MI is on the right and the group remaining in the oocyte is on the left. Each FISH signal (yellow) represents a single X chromatid. (A) Normal MI segregation of homologs results in two X signals (or one doublet) in both the oocyte and polar body; (B) intact segregation of the univalent X is evident either as two signals in the egg or in the polar body (not shown); (C) PSCS of the univalent X chromosome is evident as a single signal in both the oocyte and the polar body.