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Number of C-bands of human isochromosome Xqi and relation to 45,X mosaicism
  1. Lillian Y. F. Hsu,
  2. Sophie Paciuc,
  3. Karen David,
  4. Steluta Cristian,
  5. Ralph Moloshok,
  6. Kurt Hirschhorn
  1. The Department of Pediatrics, Division of Medical Genetics, Mount Sinai School of Medicine of the City University of New York, New York 10029, U.S.A.


    With combination of C and G banding techniques, three morphologically different types of isochromosome for the long arm of X (Xqi) have been identified, i.e. those with one C-band and symmetrical banding patterns of both arms, those with two C-bands and symmetrical banding patterns of two arms, and those with two C-bands but with asymmetrical banding patterns of two arms. The last type is a heterogeneous group with various different asymmetrical patterns. We have studied 6 cases of Xqi with C, G, and Q banding: 3 showed one C-band and symmetrical arms and all these were without 45,X mosaicism; the other 3 cases showed two C-bands, 2 of the cases having symmetrical arms and being mosaic for 45X/46,XXqi/47,XXqiXqi (those 2 were a pair of identical twins). One other had asymmetrical arms and was mosaic for 45,X/46,XXqi. Including our 6 cases, there have been a total of 30 reported cases of Xqi with C and G banding studies. Two-thirds of Xqi's were found to have 2 C-bands and one-third to have 1 C-band. Mosaicism was found in 85% of Xqi's with 2 C-bands and in only 44% of Xqi's with 1 C-band. Apparently, Xqi's with 2 C-bands have a greater tendency for anaphase lag and mitotic nondisjunction. Several possible mechanisms for the formation of the different types of Xqi's are discussed.

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    • 1 This paper was presented in part at the Annual Meeting of the Society for Pediatric Research, San Francisco, California, 28 April 1977.

    • 2 This work was supported by USPHS Grants (GM-19443 and HD-02552) and a Medical Service Grant from the National Foundation (C-155).