Chromosomes and Genes 



123 



the duplication as such. The same breakage effect is produced by 

 inversions and translocations in the absence of any duplication. But 

 the Sturtevant effect is superimposed upon the Bar effect and is due 

 to the duplication and triplication. This effect is found when the 

 following arrangements are compared, assuming only three bands 

 abc instead of six, duplication breaks being marked by crosslines, 

 each containing four Bar sections in different arrangement: 



(1) heterozygous double Bar 

 (BB/+) or BbBbBVBb 



(2) homozygous Bar 

 (B/B)"or BbBb/B^Bb 

 (See scheme, p. 121.) 



The difference is between a homozygous rearrangement with two 

 breaks in each chromosome (2) and a heterozygous rearrangement 

 with three breaks in one chromosome and none in the other (1). In 

 one rearrangement (2) we have only two breakage effects (though 

 homozygous); in the other (1), three different breakage effects 

 (though heterozygous) which seem to be additive. (The difference 

 is retained under special conditions also: the presence of an enhancer 

 for Bar described by Bonnier et at, 1947, leaves the Sturtevant 

 effect as it was.) This means that we compare the effect of a 

 heterozygous three-break, triple position effect (breakage effect) 

 with a homozygous double one. Now there is no way of predicting, 

 even for ordinary Mendelizing pairs of alleles, the quantity of a 

 heterozygous effect as compared to the homozygous dominant one. 

 If DD has an effect measured as 100, Dd may show the phenotype 

 of 100 (complete dominance) or any other value (incomplete domi- 

 nance). An exact dosage effect for dominant heterozygotes, that is, 

 50 per cent action, is possible but not necessary. If we think for a 

 moment of Bar in the old terms of a dominant gene, the knowledge 

 of the action (in terms of facet inhibition) of B/B and B/+ does 

 not permit us to draw conclusions concerning the action of BB/+, 

 except when we know that the dominance of B is strictly proportional 



