Abstract
In order to determine the presence or absence of dosage compensation in triploid intersexes (2X3A) relative to triploid females (3X3A), the rate of RNA synthesis in a segment of the X chromosomes was indirectly measured by autoradiography. Using an autosomal segment as basis for comparison it was found that, over all, there is a tendency toward equalization of the relative activity of X chromosomes in 2X3A and 3X3A individuals. When one allows for differences in selfabsorption of beta particles, it is apparent that the phenomenon of dosage compensation is incomplete. It is suggested that the present as well as other instances of dosage compensation in Drosophila can be explained if it is assumed that the rate limiting element in the transcriptional process is not the X-chromosome DNA template but a factor(s) originating elsewhere in the genome whose presence is necessary for the expression of a given sex-linked gene. Other hypotheses of dosage compensation are discussed.
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