64 



THE CELL AND PROTOPLASM 



caused by a number of pattern changes in 

 the chromosomes, especially translocations, 

 provided that one break is near the white 

 locus in the X-chromosome (Muller, Pat- 

 terson and collaborators, Schultz and oth- 

 ers). Griff en and Stone have produced 

 new chromatin rearrangements from such 

 cases by X-raying, which show as visible 

 effects many different kinds of mottling, 

 including different colors. It ought to be 

 kept in mind that the white locus which is 

 near these chromosome breaks tends to pro- 

 duce numerous multiple allelic color mu- 

 tants and further that in one of the cases 

 of spontaneous mass mutation, referred to 

 above, a series of these alleles appeared 

 (Goldschmidt). 



But this is only a small part of the facts 

 regarding patterns which result in mottling 

 and other related effects. There is another 

 set of facts, too complicated to be reported 

 in detail in a short paragraph. "We empha- 

 size therefore only the points which are im- 

 portant for our discussion (work of Muller 

 et al., Dubinin et at., Schultz, Noujdin, Pat- 

 terson et al., and others). It is known that 

 the chromosomes contain special regions of 

 heterochromatin. In Drosophila they are 

 usually called inert regions because typical 

 mutant loci are not found there. There is 

 a large inert region in the X-chromosome 

 (beside others), and the Y-chromosome con- 

 sists of this inert material. It is this mate- 

 rial which has to do with some remarkable 

 pattern-effects. If parts of this material 

 are transferred to other regions of the chro- 

 mosome by means of inversions, the result 

 is a developmental mosaicism for the effects 

 of loci brought in contact with the inert 

 material. These effects are different for 

 different regions of the inert material, and 

 there are indications that this material is 

 organized into larger blocks. But also the 

 opposite effect is observed, namely, the in- 

 hibition of an otherwise present genetic 

 mosaicism. It is claimed further that there 

 is a proportionality between the amount of 

 transferred inert material and the quantity 

 of the effect. This proportionality is espe- 

 cially important in the cases in which the 

 amount of heterochromatin determines the 



amount of dominance or recessiveness of a 

 locus (Noujdin). (We point here, by way 

 of comparison, to the other above-mentioned 

 position-effects influencing dominance.) 

 This dominance shifting effect of the inert 

 material, by the way, is combined with the 

 production of mosaicism. The same pat- 

 tern change further produces the phenotype 

 of different multiple alleles of the gene in 

 question (see above Griff en and Stone). 

 There is further an embryological relation 

 between the mosaicism and the rate of dif- 

 ferentiation (Noujdin, Surrarer), a relation 

 which also is otherwise characteristic of 

 mutant action as well as of some other posi- 

 tion effects (Goldschmidt). We add, finally, 

 that the known genetic actions of the Y- 

 chromosome (Stern), usually described in 

 terms of alleles of something within the 

 X-chromosome (suppression of bobbed, 

 etc.), are actually, as far as information 

 goes, actions of whole blocks of this chro- 

 mosome. 



Before we state our conclusions, which 

 are based upon an immense material of 

 which a selection has now been presented, 

 we may be permitted to point finally to one 

 important group of facts which have never 

 been discussed in the light of these recent 

 developments, but which might in the fu- 

 ture become of decisive importance for our 

 present problem. I mean the facts regard- 

 ing the sex-differentiating mechanism. It 

 has been proved now for almost thirty years 

 (by the Lymantria work of the author) that 

 sex-determination is the result of a balance 

 between male and female determiners, and 

 that the X-chromosomes control the two 

 alternatives of this balance by providing 

 one or two sats of determiners of one sex, 

 those of the other sex being constant. There 

 has since been a good deal of discussion re- 

 garding the question, rather unimportant 

 in itself, whether there are single sex-genes 

 or multiple sex-genes involved in this de- 

 cision. This problem has reached a con- 

 dition of impasse, as both sides (Gold- 

 schmidt and followers ; Bridges and follow- 

 ers) believe they have proved that the other 

 side has misinterpreted its experimental 

 evidence. (There are only two direct at- 



