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THE AMERICAN NATURALIST [Vol. XLIX 



To explain the slight but constant excess of doubles 

 over singles, Miss Saunders assumes that two comple- 

 mentary linked factors, X and Y, are essential to single- 

 ness, and that these factors cannot be carried by the male 

 gametes of the double-throwers, which are all xy. X and 

 Y are supposed to be so linked in the ovules that the four 

 kinds of eggs are produced, not in equal numbers, but in 

 the ratio 7XY : lXy : lxY : 7xy— or else in the ratio 15 : 1 : 

 1 : 15. Fertilization by xy pollen will give, in the former 

 case, 7XY-xy + lXy xy + lxY-xy + 7xy xy; if only 

 zygotes having both X and Y are single-flowering, only 

 the first class will consist of singles, and the doubles will 

 constitute 9/16, or 56^ per cent., of the total. Linkage 

 on the 15:1 plan would give 17/32, or 53^ per cent., of 

 doubles. 4 



For certain cases where crosses with pure singles have 

 given much less than 25 per cent, of doubles in F 2 , Miss 

 Saunders assumes the presence of a second set of two 

 linked factors, X' and Y' ; then any zygote receiving X or 

 X' together with Y or Y' is a single, and the proportion 

 of doubles is correspondingly reduced. 



Von Tschermak (1912) favors Miss Saunders's hypoth- 

 esis; he suggests the possibility of selective elimination 

 (in a dihybrid scheme), but does not consider this expla- 

 nation probable. It would seem, however, in view of con- 

 siderations stated below, that any di hybrid scheme to ex- 

 plain the usual slight deviation of the double-throwing 

 races from a 1 : 1 ratio is unnecessarily complex. 



Goldschmidt's (1913) hypothesis assumes selective 

 degeneration or sterility of pollen in the double-throw- 

 ers, and considers the case to be one of sex-linkage, class- 

 ing the slightly aberrant ratio with the known cases of 

 slight deviation in the sex-ratio in animals. He supposes 

 that this " hermaphroditic " plant is homozygous for a 

 distinct factor for femaleness (F), producing eggs all of 

 which carry this factor. He assumes that singleness is 



