242 GENETICS IN RELATION TO AGRICULTURE 



The behavior is truly remarkable when viewed in the light of modern 

 Mendelian conceptions. The number of character differences between 

 the two forms is very considerable, and the recovery of the parental 

 forms with almost unimpared fertility is so frequent that subsidiary 

 assumptions must be made to account for them on a Mendelian basis. 

 Goodspeed and Clausen, therefore, have developed the conception of 

 Mendelian reaction systems for an explanation of these phenomena. 

 According to this conception the normal functioning of a gametic or zy- 

 gotic set of factors depends upon the harmonious interrelations which 

 the factors maintain with one another. The uniform resemblance of the 

 FI hybrids of N. tabacum varieties with N. sylvestris to the N. tabacum 

 varieties is held in these cases to indicate that the N. tabacum set of 

 factors is dominant as a Mendelian reaction system to the set of factors 

 contributed by N. sylvestris. The fact that these hybrids so completely 

 resemble the N. tabacum parent indicates that the elements of the N. 

 sylvestris system are throughout unable to interact normally with those 

 in the opposed N. tabacum system. It is for this reason that a reces- 

 sive factor which is practically completely swamped in F\ intervariety 

 crosses in N. tabacum, expresses itself so strongly in the FI hybrids with 

 N. sylvestris for, if the corresponding element of the N. sylvestris system 

 were unable to interact with the elements of the dominant reaction 

 system, then it is clear that although the factor is dominant, the corre- 

 sponding character cannot possibly express itself in the individual. 



The haploid number of chromosomes in these Nicotiana species and 

 varieties is probably twenty-four. Consequently the recombination 

 series is given by the expansion of the expression (1 + I) 24 . Only one 

 gamete in 16,777,316 would carry only N. tabacum chromosomes and 

 the same proportion would hold for gametes carrying only N. sylvestris 

 chromosomes. This is on the assumption that no crossing-over occurs 

 in the formation of gametes in the FI hybrid. If crossing-over should 

 occur normally the proportion of pure N. sylvestris or N. tabacum gametes 

 would then be correspondingly reduced. The further assumption is 

 also tacitly made that there are some factor differences between N. 

 tabacum and N. sylvestris in every chromosome, which is in all probability 

 correct when we consider the striking differences between the two species. 

 Accordingly the results of the back-cross with N. sylvestris which gives 

 a relatively high percentage of what are apparently pure N. sylvestris 

 plants are exceedingly significant. Developing the reaction system 

 hypothesis, it would appear that, if the N. tabacum and N. sylvestris 

 systems display a high degree of mutual incompatibility, any gamete 

 containing elements derived from both systems would give a reaction 

 system subject to profound disturbances incident upon the inharmonious 

 relations set up between the N. tabacum and N. sylvestris elements. If 



