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: 
Q(oX)¥ :2¥ :1(0X):1 VY. 
When such males are mated to sable females, all the males in F, 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,. Bridges showed that among fifty- 
four females only fifteen gave no exceptions in Fy. 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 XXYY, 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 XXY oreven XXYY. Evidently, therefore, the presence of the 
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