80 Nature of the Genetic Material 



Serra, 1949). Prokofyeva-Belgovskaya maintains that in a series of 

 mottled position effects it can be seen that the dichromatic bands 

 adjacent to the heteiochromatin become heterochromatinized. This 

 means that instead of clear salivary bands one finds a diffuse chro- 

 matic "network" of the kind seen in tlie chromocenter. The idea is 

 that the function of the euchromatic loci is impeded by this hetero- 

 chromatization, all of which sounds to me rather crude. Other investi- 

 gators, however, could find no such cytological changes, which can- 

 not be accepted as established. Serra is more cautious: he does not 

 think of euchromatic bands changing into heterochromatin, but of 

 heterochromatin enveloping the euchromatin and hindering its func- 

 tion. This whole heterochromatization, even if it were a fact, does 

 not help us to understand why the result is mottling rather than 

 normalcy, or why euchromatic rearrangements do not result in mot- 

 tling. 



I have never doubted ( see Goldschmidt, 1946fc ) that the mottling 

 is a developmental and not a genetic or cytological phenomenon. 

 If an ordinary (euchromatic) position effect means that a locus near 

 a rearrangement break acts as if it had mutated, the mottled effect 

 could only mean that in the presence of heterochromatin the changed 

 action is not perfect. This means that the action is not completely 

 penetrant in all cells. If the white eye mutant is involved, some of 

 the ommatidial cells show the effect above its threshold of action and 

 are free of pigment (white). In others the threshold is not surpassed 

 and they are pigmented. Still others may react near the threshold 

 and form variable small quantities of pigment, depending upon the 

 breadth of the thi-eshold zone. My conclusion was that the neighbor- 

 ing heterochromatin simply weakens the effect of breakage, which, 

 because of the developmental features of pigmentation, results in a 

 mosaic action, a type which is otherwise well known in mutants 

 with varying penetrance. Recent work by E. Sutton Gersh (1952) on 

 the development of mottled eyes completely confirms this point of 

 view. Apart from the problem of position effect, this shows again a 

 generalized, quantitative action of heterochromatin upon cellular 

 processes, well in conformity with our former conclusions on the 

 function of block heterochromatin. 



These deliberations seem to be so important that a few more 

 detailed facts should be discussed. I have already mentioned that 

 loci adjacent to the euchromatic break may show more or less varie- 

 gation. It seems (literature in Lewis, 1950, and Hannah, 1951) that 

 there is a correlation between the distance of a locus from the break 



