Studies of X-chromosome inactivation in trisomies.

Previous studies of human triploidy have implicated the autosomes in the regulation of activity of X-linked genes. To determine if the influence of the extra autosomal set in triploids was attributable to a specific autosomal gene, we examined the pattern of DNA replication in females trisomic for 18 of the possible 22 autosomal trisomies. We find no evidence that a single gene or chromosome in triplicate can effect X-chromosome replication or activity. Our results suggest either that more than one locus in triplicate is required to maintain the activity of two X chromosomes, or that the influence of the extra autosomal set in enabling the activity of more than a single X chromosome is mediated indirectly, perhaps through the agency of increased nuclear size or changes in the cell cycle.