204 GENETICS IN RELATION TO AGRICULTURE 



in diagram in Fig. 92. There are two possible types of synapsis in 

 non-disjunctional males, the ordinary type of heterosynapsis in the male 

 in which Y is paired with X, in which case one Y is free, or the YY type 

 of homosynapsis in which the X-chromosome is free. Obviously, if 

 these two forms of synapsis take place according to the laws of chance 

 homosynapsis will occur twice as often as heterosynapsis. Assuming 

 this to be true the gametic series of a non-disjunctional vermilion male 

 will be as follows: 



2(vX)Y:2Y:l(vX):lYY. 



When such males are mated to sable females, all the males in FI are sable 

 and all the females are of the wild type. No exceptions, therefore, are 

 produced in F\ } but two-thirds of the daughters are non-disjunctional 

 and should give exceptions in F 2 . Bridges showed that among fifty- 

 four females only fifteen gave no exceptions in F 2 . Consequently 72 

 per cent, of the females must have been non-disjunctional, and this 

 may be regarded as an insignificant deviation from the expected value of 

 67 per cent. 



We cannot go into detail concerning any other of the numerous 

 points which have been investigated with respect to non-disjunction and 

 its attendant phenomena. That non-disjunction is not due to the pres- 

 ence of a sex-linked factor was proven by two lines of experimental 

 evidence. In the first place such a factor should have shown linkage 

 relations with the sex-linked factors and consequent crossing-over in 

 definite percentages with different loci. An extensive series of matings 

 showed, however, that non-disjunction was entirely independent of 

 linkage relations. The other line of evidence related to attempts to 

 establish pure stock of non-disjunction. These attempts failed com- 

 pletely, a fact readily explainable on the basis of non-disjunction, but 

 reconciled with considerable difficulty to the factor idea. If this were 

 not sufficient evidence, the results of cytological examination are cer- 

 tainly conclusive. Examination of a number of exceptional females 

 showed them to be of the chromosome constitution XX Y, and examina- 

 tion of regular females from non-disjunctional mothers demonstrated 

 that about half of them were XX Y, as was to be expected from theory. 

 In brief the entire series of investigations give unique support to the 

 chromosome theory of heredity, for throughout in this exceptional 

 behavior of the hereditary mechanism, the factor distribution exactly 

 parallels the unusual history of the X-chromosomes. 



From the standpoint of the inheritance of sex the investigations on 

 non-disjunction throw interesting sidelights on the relations of chromo- 

 some constitution to sex. Thus females may be of the constitutions 

 XX or XX Y or even XX YY. Evidently, therefore, the presence of the 



