No. 520] MEN DELI AN PHENOMENA 



225 



cept one produce none of a given substance in question. 



Table IV deals with that class of characters which 

 appear in heterozygotes, but not in pure races. Loche, 

 Von Tschermak, Shull and Emerson have all studied such 

 a character in beans. In certain crosses between beans 

 not having spotted seeds the heterozygotes were spotted. 

 Let us assume that the development of spotting on the 

 seed coat is due to certain metabolic activities that in- 

 volve at least two substances the production of which is 

 either a generalized function of all cell organs or is a 

 function of at least one pair of chromosomes. Let 8 X 

 represent one of the substances necessary to these reac- 

 tions, and # 2 another. Further, let us assume that in 

 order to bring about the conditions necessary for the reac- 

 tion both S x and S 2 must be present in a proportion 85 

 per cent, as great as in pure races of beans with spotted 

 seed coats. In one of the races we assume a deficiency 

 of substance S 2 , and we assume that this deficiency is 

 found in one pair of chromosomes only. In the other 

 race there is a deficiency in substance S lf and the same 

 pair of chromosomes is responsible for this deficiency. 

 Column 3 of Table IV shows 80 per cent, of the amount 

 of S 2 found in ordinary spotted races of beans, while 

 column 4 shows a similar deficiency in the substance 5, in 

 the other race. 



Now generation F x of this cross receives one of the 

 chromosomes in question from each of the parents, and 

 in the case of each substance there is a deficiency of 

 only 10 per cent. This gives us a spotted seed coat. 



In the second generation, which is not shown in the 

 table, one fourth of the progeny would be like the male 

 parent and the other fourth like the female parent, neither 

 of which has spotted seed coats, while half of the progeny 

 would be like F x with respect to spotting of the seed coat. 



While in this discussion it has been assumed that each 

 of the substances with which we dealt was produced as 

 the result of a general function of protoplasm, this is 

 not necessarily the case. When we consider the history 

 of the chromosomes it would appear very reasonable to 



