Nonfluorescent Y chromosomes. Cytologic evidence of origin.

Twelve presumptive structurally altered Y chromosomes were studied with Q-, G-, G-11, C-, Cd, and lateral asymmetric banding techniques and were compared with normal X and Y chromosomes and with an abnormal [i(Yq)] Y chromosome that exhibited intact fluorescence. Significant to this work is the fact that the Y chromosome has a small block of Giemsa-11 heterochromatin adjacent to the centromere on the long arm, while the X chromosome does not, which allows a distinction between the X- and Y-derived chromosomes. Two of the twelve altered chromosomes of either X or Y origin are small nonfluorescent rings. Each ring has a G-11-positive band of heterochromatin at the centromere, confirming Y origin. Each of the normal-length nonfluorescent presumed Ys and a Y with a fluorescent band in the center have one G-11 band at the centromere and another at an equal distance from the end of the long arm, the bands also being Cd positive, indicating that these chromosomes are pseudodicentric. The likely mechanism of origin is a break at the distal bright heterochromatin/euchromatin junction (or within the bright segment in the chromosome with the bright center band), fusion of the sister chromatids at the breakpoints, and loss of the distal segment.