Nov. 12. 1921 Genetic Behavior of Spelt Form in Crosses 359 



Castle was able to increase and decrease the pigmented area. His belief 

 then was that the variations observed in the race of hooded rats were 

 not mere fluctuations but were hereditary variations in the sense that 

 the factor for the hooded condition had undergone alterations. 



The assumption of unit-factor inconstancy, which Castle applied to 

 account for variation of pigmentation of his hooded rats of course can not 

 be applied for the analogous variations in the groupings of the spelt 

 individuals, for, if this were the case, variations in this same extent 

 should have been present among the self -fertilized population from which 

 the parental form was selected. The classes of spelt inheritance in the 

 parental strain ranged from i to 4, the mode being between classes i 

 and 2 . No departures nearly as great as those found in the homozygous 

 extracted spelts of the F3 generation were observed among this parental 

 population. The study of the F2 generation shows clearly that either 

 some modifier or modifiers were introduced by the nonspelt parent or 

 were carried by the spelt parent, but these modifiers were reduced to a 

 recessive state as a result of crossing. 



PRODUCTION OF SYNTHETIC WHEATS BY CROSSING TWO SPELTS, AND 

 SYNTHETIC SPELTS BY CROSSING TWO WHEATS 



The writers frequently have obtained synthetic spelts in interspecific 

 crosses in wheat. No indication has been observed as to the possibilities 

 of producing true wheats in crosses between two different nonwheat 

 species. It is theoretically possible, however, that such forms eventually 

 will be produced in crosses between certain kinds of spelts. This suppo- 

 sition may be explained by taking as an example the results of one of the 

 experiments discussed at length in this paper. 



In the case of cross 13126a it Avas shown in detail that two spelt factors 

 Si and S2 were involved; that the F2 segregates which bred true to the 

 spelt character were not all genetically identical; and that they were 

 composed of five genotypic forms, namely: 



v3iOiv32^2 wiS202^2 W1D1V52S2 ^1^1^2'^2 Sj^Sj0202» 



As long as these forms are allowed to be selfed, as they are in nature, 

 no wheats ever segregate; but, by hypothesis, in a number of crosses 

 between these five forms, a certain proportion of wheats are expected 

 to appear in the following manner : 



1. Crosses producing no wheats: 



SiSiSaSaXany other genotype; SiSiSoSaXSiSiSgSa; S1S1S2S2XS1S1S2S2. 



2. Cross where one out of every four F^ plants will produce 6^ per 

 cent wheats : 



*-'l^l^2^2 X v5iSiv32W2 • 



3. Crosses where half of the F^ plants will produce 6^ per cent wheats: 



^l*-'l^2^2Xv3iSiv3202; Oiv3i02S2XSiSit52S2- 



4. Cross where all F^ plants will produce 6^' per cent wheats: 



^1^1^2^2XSiSiv32»32' 



