Chromosomes and Genes 135 



it is therefore not unexpected that an unbending adherent of the 

 strictest classical theory of the gene has expressed his abhorrence of 

 such an idea. How can there be a position eflFect in the absence of 

 a gene? As we shall see below, the gene and its position are not 

 involved at all in the problem. Here only the decisive fact must be 

 stated: yellow deficiency can be obtained homozygous in a semilethal 

 condition, in which the color can still be checked. It is yellow 

 (Ephrussi, 1934; Stern, 1935). This is in no way a unique example. 

 Here are a few others: scute (Sturtevant and Beadle, 1936), white 

 (Panshin, 1941), roughest (Emmens, 1937), probably facet (Oliver, 

 1937), and a large number of loci in maize (McClintock, 1944). 

 ( In a discussion of this point I heard the objection that the + yellow 

 (+y) locus may not produce wild type but may prevent the appear- 

 ance of yellow, which appears in its absence. If this were so, the 

 mutant yellow should not be yellow but either normal or something 

 between yellow and normal. But the whole argument, if applied to 

 all the other cases, appears to be rather facetious.) 



An interesting variant of these facts has been explored by Sutton 

 (1940). She found a small terminal deficiency at the tip of the 

 X-chromosome, not containing the yellow locus. If the other chromo- 

 some contained yellow, that is, not opposite the deficiency, the effect 

 was yellow. This means that the deficiency break left of yellow acted 

 like another rearrangement break, producing a recessive yellow po- 

 sition effect. Though this belongs to the general group of position 

 effects, it is reported here because a terminal deficiency is involved, 

 meaning that no other "gene" has been brought in contact with 

 the yellow locus. Thus it must be the change in pattern itself which 

 accounts for the effect. 



It is worth while to consider thoroughly this group of facts when 

 discussing the nature of the genetic material. Taking the yellow locus 

 as an example, the phenotype yellow is produced by the following: 



1- y/y = homozygous point mutant 



2. y/ — = yellow deficiency opposite yellow 



3. — /— = homozygous deficiency for yellow 



A + + y o X, , 



4. — = button 8 case 



R + K 



5. = yellow opposite a rearrangement near the locus 



+ y 



^ R-f- , 



6. — — - = homozygous recessive position effect of rearrangement 

 K -f- 



