134 Nature of the Genetic Material 



cc. Some special features 



In the discussion of position effect some additional facts are 

 relevant which go beyond the scope of those already mentioned. A 

 rather strange fact relates to the covering effect of a duplication 

 containing the normal locus over the otherwise present two recessives. 

 A female Drosophila carrying two X-chromosomes with the mutants 

 yellow and, in addition, a fragment of the tip of the chromosome in 

 which the yellow locus is contained will be normal. But Sivertzev- 

 Dobzhansky and Dobzhansky (1933) found that if the duplicated 

 fragment has its break near the locus (yellow in the example), the 

 recessive character appears (more or less). This means that the 

 "covering" segment acts now, by a position effect, as if it were 

 mutated (to yellow in the example). Of course this is expected, 

 according to the foregoing discussion, but it is rather disconcerting if 

 considered independently. 



Another rather difficult problem has to do with deficiencies. A 

 frequently found fact is that a deficiency for a normal locus in the 

 presence of the normal locus in the homologous chromosome results 

 in a phenotype which is more or less identical with that of a known 

 mutant located in the affected region. A number of deficiencies, 

 including the vestigial locus and heterozygous with a normal chromo- 

 some, have the phenotype of a vestigial allele. Different interpreta- 

 tions are possible; for example, the normal locus is not strong enough 

 to keep the wing normal if present only in one dose. If the deficiency 

 is rather large with breakage points far away, this explanation or 

 one of a similar type suffices. If, however, the deficiency is small 

 and one or both breaks are located near the vestigial locus, one 

 could ask whether a break could produce a position effect (in this 

 case a dominant one) even in the absence of the nearby locus. This 

 would imply the question whether position effect is produced by the 

 normal locus in the presence of a break nearby, or whether it is 

 produced, more generally, by the change in the order of the constitu- 

 ents of the chromosome in a definite region (a problem which will 

 be taken up later). Thus, then, the problem arises whether a de- 

 ficiency might produce a mutant phenotype by a position effect of 

 the break (i.e., a pattern change) even in the absence of the known 

 locus. A recessive position effect, that is, a/Df(a), does not enter 

 this picture, because a recessive mutant opposite the deficiency gives 

 a haploid effect. The question must appear preposterous to anybody 

 who considers position effect in terms of the position of a gene, and 



