344 INTRODUCTION TO CYTOLOGY 



hypothesis does not assume that any one factor produces a particular 

 character directly and by itself, but only that a character in one organism 

 may differ from a character in another because the sets of factors in the 

 two organisms have one difference." "It can not . . . be too strongly 

 insisted upon that the real unit in heredity is the factor, while the charac- 

 ter is the product of a number of genetic factors and of environmental 

 conditions" (Morgan et at, 1915, pp. 210, 212). 



The abundant opportunity for the formation of new factor combina- 

 tions should be noted in this connection. An organism with four pairs of 

 chromosomes in its body cells, and only one pair of factors in each chromo- 

 some pair, could form, as the result of the independent distribution of the 

 four pairs of chromosomes, gametes with as many as 16 different geno- 

 typic constitutions (Fig. 134). Such a diversity being present in the 

 gametes of both sexes, this means that more than 200 different com- 

 binations are possible at fertilization. The 12 pairs of chromosomes in 

 man may in the same way form several million such combinations. Since 

 there is good reason to believe that each chromosome carries more than 

 one factor the number of variations actually produced by these means is 

 almost incalculable. This subject will be pursued further in the chapter 

 on Linkage (Chapter XVII), where the evidence for the presence of many 

 factors in a single chromosome will be presented and the consequences of 

 this condition pointed out. 



MUTATION 



Although opinion is divided over the question of the real nature of the 

 phenomenon of mutation, particularly in (Enothera Lamarckiana, one 

 school (deVries et at.) holding that it represents the actual origin of new 

 forms, and another (Bateson, Davis, Lotsy) regarding it as the result of 

 segregation in an organism of hybrid constitution, the observed facts in 

 either case are nevertheless very significant with respect to the chromo- 

 some theory of heredity. The mutations observed to arise from (Enothera 

 Lamarckiana fall into two general classes: first, those accompanied by 

 alterations of the normal chromosome number (seven pairs), and second, 

 those in which the number undergoes no change. 1 



Mutations Accompanied by Changes in Chromosome Number. — It 

 is to be noted first of all that the mutants belonging to this class do not 

 behave in a typically Mendelian fashion when bred to other forms, and 

 that this is correlated with the serious disturbance of the chromosome 

 mechanism. (Enothera mutants with many abnormal chromosome num- 

 bers have been observed; Gates, for example, found them with 15, 20, 

 21, 22, 23, 27, 28, 29, and 30 chromosomes. 



1 Our knowledge of the cytology of the Oenotheras is due mainly to the researches 

 of Gates, Davis, Stomps, and Miss Lutz. 



