WHITE— STUDIES OF INHERITANCE IN PISUM. 581 



or more different factors with a variety lacking these factors, mak- 

 ing the cross a sufficient number of times to insure a large F^ popu- 

 lation (4' or 16,384 individuals at least) or by making all the pos- 

 sible combinations of the seven-factor pairs through separate cross- 

 ings. In Fo, in the former case, if each factor is inherited inde- 

 pendently of all the others and large enough numbers of progeny 

 are grown, there should be 128 F^ combinations which remain con- 

 stant in F3 and later generations and 2,187 combinations all together 

 (60), each of which would be represented in a definite proportion of 

 the progeny. Each of the seven factors should be present in ap- 

 proximately three fourths and absent in one fourth of the total 

 offspring. If 8 factor differences were involved, the various numer- 

 ical terms would be proportionally increased. But in the event that 

 a cross involving 8 factors did give the theoretical expectation for 

 the independent Mendelian segregation of eight pairs of factors, 

 the chromosome theory, as at present held, would either be dis- 

 proven or modified, for there would be only seven pairs of chromo- 

 somes involved in carrying the eight pairs of factors through the 

 mazes of the maturation divisions, where segregation is believed 

 generally to take place. 



More accurate data on this subject are obtainable by back- 

 crossing the Fi hybrids with the recessive classes, but back-crossing 

 in peas on a large scale is impracticable, as so few seeds are obtained 

 from each cross. The large size of pea chromosomes, as compared 

 to those of Drosophila, may be assumed to indicate, on present 

 theories, a looser linkage of the factors of each group, as compared 

 with the close linkage of the Drosophila groups. This loose linkage, 

 if it exists, increases the difficulties of classifying the factors in 

 groups and in determining their relation to each other within the 

 group. 



Inheritance studies on Pisum so far have disclosed only four 

 linked groups of factors (ACEGcL^Lf, RTl, GO, SBl or SW), and 

 several other doubtful groups in which some of the factors are not 

 as yet clearly delineated. In the first group, the evidence is com- 

 plete enough to show the coupling is absolute except for the factor 

 Lf and hence for simplicity's sake, the first five factors may be 

 regarded as one. G and O also appear to be partially coupled, 



