Regulation of Gene Action — Dosage Compensation 



489 



the same degree of phenotypic effect in both 

 males and females (probably representing 

 dosage compensation) than we find in the 

 arm homologous to the melanogaster III L. 13 

 As a working hypothesis we can suggest 

 that the suppression involved in dosage com- 

 pensation in Drosophila is closely associated 

 with messenger RNA. The messenger RNA 

 (or the protein product) of compensator 

 genes could either interact with and inac- 



i:i The preceding discussion of dosage compensa- 

 tion in Drosophila to a great extent follows the 

 work of H. J. Muller (1950). 



tivate messenger RNA of the genes they 

 compensate, or they could act directly upon 

 the gene to be compensated interfering with 

 its production of messenger RNA. This 

 suppression might be based upon the mes- 

 senger RNAs of all dosage compensators 

 having some common nucleotide sequence 

 (p. 461), either the complement or the 

 equivalent of a deoxyribotide sequence in 

 the locus being suppressed. Perhaps the 

 operator genes of the loci being compensated 

 are suppressed by the regulator dosage com- 

 pensator genes. 



SUMMARY AND CONCLUSIONS 



Dosage compensation for X-linked genes is accomplished by suppression of gene 

 function. 



In human beings, in mice, and probably in all mammals having sex chromatin, this 

 suppression is associated with heteropycnosis and, hence, chromosome coiling. Al- 

 though some differentiation in a chromosome is temporary, dosage compensation and 

 other nuclear phenomena demonstrate that the chromosome can become more or less 

 permanently fixed in some way related to its function. 



In Drosophila, another mechanism is responsible for the regulation of gene action 

 leading to dosage compensation. It is hypothesized that this process involves the 

 transcription, translation, or protein products of messenger RNA; perhaps dosage com- 

 pensator genes are regulator genes which control operator genes of the loci being 

 compensated. 



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