Chromosomes and Genes 131 



silver, if both present, cannot be seen. However, in the homozygous 

 combination of the yellow inversion with bran the wings are soft blis- 

 tered, which means that one of the effects of the silver locus now 

 becomes visible in the combination as a position effect of the second 

 break. 



We can now proceed to show that position effects closely parallel 

 the actions of point mutants and, therefore, are indistinguishable 

 from them if no special effort is made. Actually, the fact that about 

 half of the mutants described for Drosophila (including multiple 

 alleles) have turned out to be position effects illustrates the point. 



In the present discussion we mean, by mutants, bona fide point 

 mutants, that is, in Drosophila, mutants with normal salivary chromo- 

 somes in the proper section. For other organisms, only gross re- 

 arrangements may be distinguished with certainty from point mutants, 

 since small ones, in terms of salivary chromosome size, cannot be 

 checked. The following are some of the parallel actions between 

 mutants and position effects that have been registered: 



1. Mutants may act as dominants or recessives or anything in 

 between. At the same locus in a series of multiple alleles, dominants 

 and recessives may occur, although such dominants are rather sus- 

 pected of being position effects of visible rearrangements (as also 

 actually proved in some, e.g., eyeless dominant). Dominant mutants 

 are frequently homozygous lethal. Some of these, like Beaded and 

 Stemopleural in Drosophila, have normal salivary chromosomes as 

 far as is known, but we must be cautious in assessing this fact. A 

 great many homozygous lethal dominants in Drosophila have turned 

 out to be position effects of visible rearrangements. Many of the 

 Minutes have been shown to be deficiencies, while others have normal 

 salivaries. Since, as we shall see, genetically proved deficiencies with 

 normal salivaries exist, we must be cautious. However, one fact is 

 certain: just as dorninant and recessive mutants exist, both types of 

 position effect are known, and wherever a phenotype is known, both 

 as mutant and as position effect, the two may be (but are not al- 

 ways) phenotypically identical ("may be" meaning that the position 

 effect may also act like a multiple allele of the locus). 



2. Mutants frequently show combination effects. Drosophila eyes, 

 in the presence of the mutants brown and vermilion simultaneously, 

 are white, a fact for which the biochemical work has given the 

 explanation. Kikkawa (1938) found that the combination of the 

 position effect Plum ( next to the brown locus and a variegated effect ) 

 together with vermilion also produced white. This fact has the 



