Summary
The chromosome replication pattern of a man with 49,XXXXY was analyzed using 3H-thymidine and autoradiography as well as BrdU and acridine orange. The former technique showed a highly irregular replication pattern; the latter revealed one early replicating X chromosome, and the other three more or less asynchronously replicating. Two hypotheses seem to explain best the abnormal phenotype of males with an XXXXY sex chromosome constitution: (1) The number of the always active regions (tip of Xp) and of the possibly always active regions (the Q-dark regions on both sides of the centromere) is increased from one to four. (2) The replication pattern of the late-replicating X chromosomes is highly asynchronous, which might affect the phenotype. The possibility that more than one X chromosome might remain active in some cells, an even more abnormal and obviously deleterious situation, is still open.
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Sarto, G.E., Otto, P.G., Kuhn, E.M. et al. What causes the abnormal phenotype in a 49,XXXXY male?. Hum Genet 76, 1–4 (1987). https://doi.org/10.1007/BF00283041
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DOI: https://doi.org/10.1007/BF00283041