162 Linkage and Crossing Over 



when they enter in the repulsion phase, the Fo ratio becomes 

 2VBu:lVbu:lvBu:0vbu. The results which Shull ob- 

 tained can be interpreted on the basis that these genes are linked 

 and that they are so completely linked that no crossing over 

 took place. Actually, as subsequent tests showed, linkage was 

 not complete, but these genes were so close together that the 

 chance of a break of chromatids in the region between them 

 was very small. In a family of only 108 plants, no crossovers 

 occurred. Had the family consisted of 10,800 plants, a small 

 percentage of crossover gametes would probably have been 

 produced. 



It is a rule that when linkage is complete and when the 

 original cross is in the repulsion phase, the F2 ratio will be 

 2:1:1:0. This is true when linkage is complete in only one 

 sex as well as when it is complete in both, as has been shown 

 many times in Drosophila. Let us a^ain consider the curved 

 and speck genes. The gametes of the female Fi fly, if the cross 

 w^as made in the repulsion phase, are 0.3 C Sp : 0.7 C sp : 0.7 

 c Sp : 0.3 c sp, but since linkage is complete in the male, the 

 sperm are in the ratio of 0.5 C sp : 0.5 c Sp. When two 

 C sp / c Sp flies are mated, the offspring are 2 C Sp : 1 C sp : 

 1 c Sp : c sp. This principle has been widely used when a new 

 gene in Drosophila has been discovered to determine with which 

 of the many known genes this new gene is linked. 



Complete Linkage and Multiple Alleles 



Although a 2 : 1 : 1 : ratio may be obtained in the F2 from 

 a cross involving completely linked genes in the repulsion phase, 

 the same ratio will also be found in certain crosses involving 

 multiple alleles. Therefore, it is sometimes difficult to determine 

 whether a particular ratio is the result of completely linked 

 genes or multiple alleles. Sometimes further extensive studies 

 have resulted in the appearance of a few crossovers, thus show- 

 ing that linked genes are involved which are not completely 

 linked but nearly so. Sometimes such extensive studies are not 

 practical and it is impossible to decide from the data available 

 the cause of the ratios. It is sometimes assumed as a tentative 

 hypothesis that multiple alleles are the cause if the three types 

 are merely differences in the same part of the organism but 



