| 1A - 6F2 | 6F2 - 1A | 20 - 7A1 | 7A1 - 20 . |
An apparently completely stable, compound-ring-X chromosome; cannot produce single-X chromosome derivative by heterochromatic exchange.
C(1)TR94 progenitor marked with y cv v sd . y sn g Apparently arose by a process describable as reversed crossing over in region 6F2 - 7A1. Current versions of this chromosome have apparently opened, since they are no longer ring-shaped in metaphase. Shown to have separated at 13E by Traverse and Pardue (1988, Proc. Nat. Acad. Sci. USA 85: 8116-20) such that the new order is 13E - 7A1 | 7A1 - 20.1 - 6F2 | 6F2 - 1 | 20 - 13E | 13E - 20.1 - 6F2 | 6F2 - 1 | 20 - 7A1 | 7A1 - 13E which are interconvertable by exchange between regions 20 and 13. The newly terminal ends at 13E have acquired moderately repeated sequences (He-T DNA) ordinarily encountered at telomeres and in the chromocenter (Traverse and Pardue). Transmission of C(1)A is reduced owing to the fact that half of meiotic exchanges lead to the production of dicentric chromosomes.
The C(1)A chromosome has spontaneously opened in polytene region 13E to produce two new telomeres. Each of the new telomeres has acquired HeT-A DNA sequences.