154 Outline of Genetics 



fused with one polar) and colorless, sugary in part (from independ- 

 ent polar). 



These critical experiments served to disprove Webber's prop- 

 ositions and proved that the normal program of double fertiliza- 

 tion is invariable in corn. The occurrence of the occasional 

 anomalous mosaic grains, however, remained to be explained. 

 "Somatic mutation" was invoked by some as an explanation, but 

 proved unsatisfactory for a number of reasons. 



Emerson (5) has finally obtained critical evidence which 

 indicates a very satisfactory explanation of the phenomenon. 

 The factor wx for waxy endosperm {Wx, corneous endosperm) 

 is known to be carried on the same chromosome with the C factor. 

 A cross was made between a colorless, waxy female parent, c-wx 

 c-wx, and a red corneous male parent, C-Wx C-Wx (the R factor 

 being present in both parents). The resulting triploid endosperm 

 was of the formula c-wx c-wx C-Wx. If non-disjunction (passing 

 of both halves of a divided chromosome to one pole) occurred in 

 connection with the third of these chromsomes, one of the result- 

 ing nuclei would be diploid for this chromosome set, c-wx c-wx, 

 and the other tetraploid, c-wx c-wx C-Wx C-Wx. Endosperm 

 produced by the former should be colorless, waxy; endosperm 

 produced by the latter should be red, corneous. Emerson 

 obtained aberrant grains which were of exactly this constitution, 

 the colorless areas being at the same time waxy and the red areas 

 corneous. This experiment, considered together with the pre- 

 vious ones, indicates that occasional non-disjunction is the expla- 

 nation of these aberrant grains. 



(The frequency of these particular aberrant grains is one in 

 423, and one may expect non-disjunction to take place in connec- 

 tion with some one chromosome in the corn endosperm in about 

 one of every fourteen grains. Direct cytological demonstration is 

 to be hoped for. Non-disjunction is known to occur at times 

 elsewhere in the plant and animal kingdoms. Possibly the trip- 

 loid nature of endosperm furnishes an especially favorable condi- 

 tion for its occurrence.) 



This fascinating series of experiments shows how features of 

 the morphological and cytological program in a plant may be 

 demonstrated in a very convincing way through the indirect evi- 



