Sex chromosome structure diagrams related to Cross A (XY* model ). Sections of the sex chromosomes have been deleted to allow expansion of other regions for illustration. For the WT sex chromosomes, the PAR is illustrated in three sections†, one of which contains Sts. The minute + subregion of the NPX (<1% of the total NPX), adjacent to the PAR, is shown as a bracketed region within the entire NPX for the X and XXY* chromosomes, and in yellow for the Y* and Y*X chromosomes. The Y* chromosome has an X centromere and the plus (+) region of NPX adjacent to a unique duplicated PAR that nevertheless lacks Sts (ΔPAR). The X centromere (X CEN) is the functional centromere, whereas the Y centromere is inactive (Y CEN). The Y* chromosome undergoes abnormal recombination with the X chromosome to produce (1) the long XY* chromosome that is an end-to-end fusion of NPX and NPY with the ΔPAR lacking Sts and (2) the minute Y*X chromosome with a normal PAR and the plus (+) region of NPX. The gene map for the Y*X illustrates the approximate positions of genes transcribed from the forward strand (black rectangles) or reverse strand (lines), with the centromere (orange) and the plus (+) region of NPX (yellow) color coded. In effect, Y*X is an X chromosome with a massive NPX deletion running from a region thought to be just proximal to Msl3 and ending within the Sstx/Spin2 multi-copy cluster adjacent to the telomeric X centromere [22, 72, 118–120]. The dividing line between NPX+ and the rest of NPX is not known precisely but is on the centromeric side of Msl3 . This figure updates and corrects earlier versions [50, 121].
† The PAR sections A–C were originally defined by mapping using the XY* model : A, the terminal section of Mid1/Fxy (including the last three exons) that lies within the PAR ; B, the region that contains the multi-copy locus DXYHgu1 ; and C, the region containing Sts . It is now known that Erdr1 is located in the PAR just distal to Mid1 and is retained in Y* and XY*—thus, it maps to the PAR region B . On the other hand, Asmt is absent in Y* and XY*—thus, it maps to the PAR region C . Sts is thought to be distal to Asmt, but this has not been confirmed