Figure 3.

Homologies between the human X chromosome and chicken autosomes. a, Plot of BLASTZ sequence alignments between the X chromosome and chicken chromosomes 1 (red) and 4 (blue). Grey bar centred at approximately 60 Mb shows the position of the X centromere. Only the relevant section of each chicken chromosome is shown (see Mb scale at left for chromosome 1 and at right for chromosome 4). A schematic interpretation of the homologies shows the XAR and XCR as red and blue bars, respectively (see Fig. 1). Homologies at the ends of the XAR are indicated with arrows and are expanded in b. b, (Top) Genes at the ends of the human XAR. Genes from distal Xp (magenta arrow in a) are in magenta and genes from Xp11.3 (black arrow in a) are shown in black. (Bottom) Arrangement of the orthologous genes on chicken chromosome 1. A hypothetical ring chromosome, with the equivalent gene order to that observed in the chicken, is indicated by the curved, dotted red line. Recombination between one end of the established X chromosome and the ring chromosome at the arrowed position could, in a single step, have added the XAR and created the gene order observed on the human X chromosome.