Genic and Non-genic Parts of the Chromosome 79 



for a multitude of mutations, which gradually established themselves 

 in the new chromosomes. Through the accumulation of such muta- 

 tions changing polygenes into major genes, some of the heterochro- 

 matin may have become transformed into euchromatin. It is also 

 possible that from tlie crowd of identical polysomes distinct pairs 

 may have become differentiated." I do not think that this type of 

 phylogenetic speculation is very difiFerent from that of Haeckel's day. 



One more group of facts should be interpreted similarly, namely, 

 the mottling position eflFect of heterochromatic breaks. Position eflFect 

 will be discussed later. Here it suffices to say that a rearrangement 

 break near a known locus produces the phenotype of a mutant of 

 that locus, the position eflFect. If the rearrangement contains a break 

 within chromocentric heterochromatin which thus becomes adjacent 

 to a euchromatic section, the position eflFect does not reproduce the 

 complete phenotype of a nearby mutant but only a part of it spatially 

 so that a mottled condition appears. If it is a position eflFect for the 

 locus of the normal allele of white, the eye is not white but white 

 and wild color (also other colors) mottled. The detailed facts have 

 been reviewed by Hannah (1951) and Lewis (1950); the original 

 discovery was made by Muller (1930). If such a heterochromatic 

 rearrangement is found near a number of closely located loci, all of 

 them may show the mottling ( variegation, mosaic ) . The grade of this 

 variegation is influenced by addition or subtraction of block hetero- 

 chromatin, and it is aflFected also by neighboring intercalary hetero- 

 chromatin. Another agent aflFecting the grade of mottling is a 

 rearrangement with heterochromatic breaks which are independent 

 of the locus of position eflFect (Schultz, 1941). However, these and 

 other comparable features are more or less expected, as it is generally 

 known (Goldschmidt and Gardner, 1942; Gardner, 1942) that re- 

 arrangements can afiFect the phenotype of independent mutants. Also, 

 temperature aflFects the variegation in a rather irregular way (E. Sut- 

 ton Gersh, 1949), which again is expected. (Many other details will 

 be mentioned below.) 



The question arising from these facts is: Why does the presence 

 of block heterochromatin adjacent to a position eflFect break produce 

 mottling? A number of authors since Demerec and Slizynska ( 1937 ) 

 have assumed that here the meaning of position eflFect is revealed, 

 namely, the production of mutation near the break. Other purely 

 genetic interpretations have been tried. Cytological interpretations 

 also have been suggested, based upon the "heterochromatization" of 

 euchromatic parts ( Prokofyeva-Belgovskaya, 1947; also accepted by 



