LINKAGE 69 



also been found in other forms than Drosophila, but 

 in these cases the determining conditions and their 

 effect on the various linkage values have not as yet 

 been discovered. 



LINKAGE IN OTHER ANIMALS AND IN PLANTS 



Since the discovery in 1906 of linkage in sweet peas 

 many cases have been found in animals and in plants. 

 In sweet peas themselves two groups of linked factors 

 are now known, one containing three pairs of factors 

 and the other three or possibly four. In garden 

 peas there are two pairs of linked factors and two 

 other cases that are doubtful; in the primrose there 

 is a group of five pairs of linked factors; in the snap- 

 dragon there is a group of three pairs; in stocks there 

 is a group of three or probably four pairs. In animals, 

 linkage, aside from sex linkage, has been discovered 

 in only one form besides Drosophila, viz., the silk- 

 worm, in which Tanaka has found that several linked 

 factors are present, i.e., four allelomorphs in one 

 locus linked to two allelomorphs in another locus. 

 There are, it is true, several other cases in which the 

 evidence leads one to suspect that linkage occurs, but 

 these are too uncertain at present to be included in 

 the list. In all the above cases the linkage is "par- 

 tial," that is, a certain amount of crossing over takes 

 place, at least in one sex. 



There are a number of cases of sex linkage, which, 

 being only a special case of linkage, undoubtedly 

 belong in the same category, but the amount of cross- 



