MENDELIAN FACTOR DIFFERENCES VERSUS 

 REACTION SYSTEM CONTRASTS IN 

 HEREDITY. II 



T. H. GOODSPEED and R. E. CLAUSEN 



The literature on species crosses shows clearly that all 

 gradations occur as regards fertility from complete steril- 

 ity to what is apparently complete fertility. Recombina- 

 tions may, therefore, in some cases, occur freely between 

 species that appear to be distinct. Lotsy (1913) in par- 

 ticular has shown this to be true for a number of species 

 crosses in Antirrhinum and doubtless such instances 

 might be multiplied considerably. But even in these cases 

 the behavior of the hybrid progeny in subsequent genera- 

 tions indicates that there is a possibility that some of the 

 recombinations do not form functional reaction systems 

 because of the discordant elements they possess. This is 

 shown in the entirely new characteristics which a certain 

 portion of the population may exhibit and in the peculiar 

 ratios which are sometimes obtained in segregation. Such 

 results only bear out more completely the conception that 

 for any recombination of elements to be completely func- 

 tional they must together form a harmonious reaction 

 system. Even Detlef sen's (1915) results with the cavy 

 species cross which gave in F^ sterile males and fertile 

 females may be brought into line with such a physiologi- 

 cal conception perhaps better than by trying to account 

 for it on the basis of any definite number of Mendelian 

 factors. At least in the cavy cross the attempt to account 

 for the results in this latter manner demonstrated that 

 the number of factors concerned must be relatively large, 

 and this is precisely what would be expected on the basis 

 of recombinations involving whole choromosomes or sec- 

 tions of chromosomes carrying with them, perhaps, many 

 discordant elements. If we recall the observation that 

 no crossing-over occurs in the sex heterozygote (Morgan 



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