GENETIC CONTROL OF CELL INTEGRATION 



341 



FIGURE 11.18. Diagram of a Drosophila salivary chromosome showing the insertion 

 of a segment of the X chromosome into the heterochromatic chromocenter region of 

 the fourth chromosome (from Sutton, 1940, Genetics, 25:534). 



When this occurred, the position effect was lost and the wild-type ex- 

 pression reappeared. Apparently then, the effect does not involve muta- 

 tion as commonly understood, since it is fully reversible. A beautiful 

 example of reversibility was provided by Catcheside in his analysis of a 

 position effect at the P locus associated with a translocation in the 

 plant Oenothera. By studying the movement of the P locus in and out 

 of the translocation by crossing over, he correlated the instability of 

 expression with the position of the locus. 



Virtually no understanding of the V-type position effect is yet at hand, 

 although many of the observations are almost 30 years old (which is very 

 old in the field of genetics). The principal features of this phenomenon 

 are: (1) the reversible repression of gene expression, and (2) the spread- 

 ing effect. 



About 10 years ago the first reports were published by McClintock 

 of an intensive analysis of genie instability and the control of expression 

 of one chromosomal element by another, in maize. Her studies have 

 provided an opening wedge for a new attack upon the position effect 

 phenomenon, and upon the more general problem which it reflects: gen- 

 etic control devices operating at the chromosome level. In the brief 

 account which follows, it will be impossible to do justice either to the 

 analytical power of the experiments behind these results or, indeed, to 

 the results themselves, which are complex, comprehensive, and diverse, 

 and have not been published in extenso. 



In the early stages of this investigation, McClintock discovered two 

 chromosomal elements with extraordinary properties. One of them, 

 called Ds (for dissociation) could induce chromosome breaks and muta- 

 tions at any position in the chromosome where it was located, and could 



