124 GENETICS IN RELATION TO AGRICULTURE 
Taste XXVI.—Linkacr or VERMILION, SABLE, AND Bar In DrosopHILa 
Single cross-overs between 
A Non- aad Double 
rameter | cross-overs | Vermilion and | Sable and peat Se 
sable | bar 
Grayered normale. vases ne 755 110 140 | t 
Gray vermilion normal......... | 734 92 151 1 
Sable red normals), .a..26 of2ce 724 97 131 4 
Grayeredibatwan act create: 845 87 126 4 
Gray vermilion sable.......... | 608 | 80 | 123 | 3 
Gray vermilion bar............ 800, | 95 | 129 1 
pablemedsbare. seocas ee eee 665 81 107 1 
Vermilion sable bar............ | 641 74 | 108 | 3 
Motalske sete terre rep ce ee Go) |e dy015 ee) ee 
Percentages emer! 76.7 | 9.53 | 13.49 0.28 
B 
Cc 
A D F 
Fia. 57.—Diagram to illustrate interference in crossing-over. 
would protect the loci A, C, and D on either side of it from crossing-over 
because there would be no close twisting of the chromosomes at these 
points. As we move on toward £, however, the frequency of double 
crossing-over would become greater and greater until at E where the 
modal length of loop was attained double crossing-over values approach- 
ing those expected on the basis of pure chance would be obtained. 
Muller has actually shown that such conditions are fulfilled in his 
twelve-point experiments and he has been able to plot a curve showing 
that the observed frequency of double crossing-over gradually increases 
until when the modal length is reached the curve coincides with that 
based on pure chance. Thus we see again how another point of attack 
has lent support to the conception that the factors are arranged in a 
linear series and that the linkage relations of factors are referable to the 
mechanical consequences of relative positions in the linear series. 
Bridges points out that interference stands in about the same relation 
to linkage as linkage does to free Mendelian assortment. Also that the 
development of the idea of interference is an illustration of the advantage 
of the chromosome hypothesis. The existence of this phenomenon was 
Digitized by Microsoft® 
