Nov. li, 1921 Genetic Behavior of Spelt Form in Crosses 357 



As the presence of multiple factors is entirely out of question, it being 

 proved in this case that the parents of this cross differ in only one factor 

 for spelting, the following explanation may be given to account for these 

 variations. One or more sets of modifiers furnished by the spelt, by 

 the wheat, or by both parents seem to be present where each set of 

 modifiers was in a homozygous dominant condition in one parent and in 

 the alternative condition or absent in the other parent. These modifiers 

 in the presence of the S factor tend to intensify the spelt character. 



An example may be given to illustrate the effect which a modifier may 

 produce in a spelt X wheat cross. The modifier which may cause dilu- 

 tion of spelting may be represented by the factor D and may be assumed 

 to be carried by the wheat parent. (If this factor were contributed by 

 the spelt parent, the latter would have been a dilute spelt, which was not 

 the case in these crosses.) The wheat parent will then be represented 

 by ssDD and the spelt parent by SSdd. The genotypic forms of the 

 successive generations will be as follows : 



12 spelts. 4 wheats. 



This represents a ratio of 3 spelts of different grades to i wheat. 



If we assume that the nature of the modifier were to produce intensifi- 

 cation of spelt inheritance in the presence of factor S, which in this case 

 may have been carried either by the spelt or by the wheat, we will have, 

 in the F, generation, 3 intense spelts, 6 semi-intense spelts, 3 normal 

 spelts, and 4 wheats. 



Some of these spelts (intense, normal, or dilute) will breed true to 

 those conditions ; others will produce some or all of these forms in different 

 proportions as expected on the factorial hypothesis. If more than one 

 set of modifiers are present the types and their proportions naturally 

 become rather complex. 



If, in the crosses 13255a and 13260a, a diluting modifier has been intro- 

 duced, we would occasionally expect among the spelts (homozygous or 

 heterozygous) some which are grouped in the dilute speltoid classes. 

 The F3 population of i3i26a-5 and -25 (Table V); i3255a-26 (Table 

 IX) ; i326oa-3, and -14 (Table X) ; and a number of others represent 

 such cases. The progeny of i326oa-2o, -6, -10 and others may represent 

 spelts carrying some intensifying factor. 



