LINKAGE 69 



also been found in other forms than Drosophila, but 

 in these cases the determining conditions and their 

 effect on the various Hnkage values have not as yet 



been discovered. 



» 



Linkage in Other Animals and in Plants 



Since the discovery in 1906 of linkage in sweet peas 

 man}^ 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 several forms besides Dro- 

 sophila, viz., in domesticated poultry by Goodale, in 

 pigeons by Cole, in rats and mice by Castle, in the 

 silk-worm moth by Tanaka, and in Apotettix by 

 Nabours. 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 " partial," 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- 



