Chromosomes and Genes 143 



This is hardly an explanation but rather a circumscription of the 

 facts arising from the embarrassing realization that the theory of 

 the gene does not seem to work here. 



Another explanation, which is used by Lefevre, Jr., Ratty, and 

 Hanks (1953) in a very important study of the N phenomenon, is 

 the assumption of invisible submicroscopic deletions. If the deficiency 

 effect, in spite of normal salivaries, were found for only one locus, 

 we could agree. But if a number of loci over a considerable stretch 

 of the chromosome show the haploid effect, a whole string of simul- 

 taneous submicroscopic deletions would be required, which is rather 

 improbable. We know only the following situations in which a reces- 

 sive mutant locus produces its effect in a single dose: (1) when it is 

 hemizygous, that is, opposite a deficiency or in the X-chromosome of 

 the hemizygous sex (a/—); (2) when powerful dominance enhancers 

 have been selected or introduced by crossing (A/a + D.E.); (3) when 

 the mutant locus is opposite a rearrangement with position effect 

 (a/R(-f-)). The first two are excluded in the present case. The only 

 explanation based upon known facts is that the Notch chromosome 

 contains a rearrangement with a position effect of varying power, 

 affecting one or more loci near the break: since no rearrangement is 

 visible (though N position effects via visible rearrangements are 

 known), we must conclude that here the Notch effect is produced by 

 an invisible rearrangement of a type which affects a considerable 

 stretch of the chromosome. 



bbb. Stickiness and mutators. — A very different group of facts 

 is also very suggestive. Beadle (1932) found mutants in maize which 

 affect details of synapsis and chromosome behavior. One of them 

 makes the chromosome sticky and results in special configurations 

 in meiosis. In such a sticky line a great increase in both rearrange- 

 ments and point mutations was found. It is easy to understand the 

 increase in rearrangements, as stickiness should produce breaks dur- 

 ing the synaptic movements and otherwise. However, the increase 

 in point mutations makes sense only when these are unrecognizable 

 rearrangements, since it could hardly be assumed that stickiness pulls 

 out some side chain from a gene molecule. I have stressed this point 

 repeatedly in former discussions of the subject. It seems that Resende 

 (1945) has independently come to a similar conclusion. He calls 

 it chromosomic agglutination, which he studied in many materials 

 (see Pinto-Lopes and Resende, 1949). His opinion is that rearrange- 

 ments occur only when two chromosomes happen to be near each 

 other and when the kalymma is depolymerized, thus removing the 



