650 VERMILION-DEFICIENCY 



these vital genes had given rise to the lethal effect, and the deficiency 

 for the vermilion locus rendered a female carrying one such chromo- 

 some virtually haploid for vermilion. A female, one of whose X 

 chromosomes carries vermilion-deficiency and the other the reces- 

 sive vermilion gene, shows the vermilion character as does a male 

 carrying the vermilion gene in its single X chromosome. From the 

 facts in certain other cases of deficiency, especially Notch 8 (N 8),^ 

 it is probable that the condition is not as simple as just represented, 

 but that the vermilion-deficiency has some positive action, in some 

 ways comparable to, but more extreme than, the action of the ver- 

 milion gene. 



Linkage Tests of the Extent of the Deficient Region and the Disturbance 

 in Crossing Over in Adjacent Regions. 



The work with the first deficiency^ had shown that there was prob- 

 ably no crossing over within the deficient region, and that the chromo- 

 some map was shortened by an amount equal to the length of the 

 deficient region. In certain other cases of deficiency, notably dachs- 

 deficiency,^ it has been found that besides this shortening equivalent 

 to the length of the deficient region there is extensive alteration in the 

 amounts of crossing over in neighboring regions of the chromosome. 

 This disturbance seems to take the form of a marked decrease in the 

 amount of crossing over in the immediate vicinity of the deficiency 

 with perhaps slight increases in more remote regions. Such an 

 effect would follow from disturbed synaptic relations and would 

 seem to argue that deficiency involves a real contraction of the de- 

 ficient chromosome, with resultant "puckers" at synapsis, rather 

 than merely an inactivation. 



The first step taken was to test whether there was marked reduc- 

 tion of crossing over in the region to the left of the supposed de- 

 ficiency. The vermilion forked and lethal forked crossover values 

 had already shown that there was probably not much disturbance 

 in the long region to the right of vermilion. The recessive mutation 

 facet whose locus is at about 2.6 was used to control the left end. 



^ Mohr, O. L., Genetics, 1919, iv, in press. 



^ Bridges, C. B., and Morgan, T. H., Carnegie Institution of Washington 

 rublication, 278, 1919, pt. ii. > 



