INHERITANCE OF SEX AND RELATED PHENOMENA 201 
Y-chromosome pairs with an X-chromosome, the free \-chromosome 
then goes as often to one pole as the other and this results in the pro- 
duction of equal numbers of X, XX, YY, and Y eggs. This set of re- 
lations is shown in diagram in Fig. 91, which illustrates the phenomena 
exhibited in the production of gametes by a vermilion non-disjunctional 
female. From experimental evidence it has been determined that 
homosynapsis, 2.e., pairing of the two N-chromosomes, takes place in 
84 per cent. of cases in non-disjunctional females and heterosynapsis, 
pairing of an X- with a Y-chromosome, in 16 per cent. of cases. A non- 
disjunctional female, therefore, will produce four types of eggs in the 
following proportions 
4(uX) (0X) :4Y :46(0N) :4600X)Y. 
When a vermilion non-disjunctional female is mated to a red male, the 
F, consists of about 46 per cent. each of red females and vermilion males 
and about 2 per cent. each of further exceptions, vermilion females and 
red males. Non-disjunctional females are, therefore, characterized by 
the production of further exceptional offspring to the extent of about 
4 per cent. This type of non-disjunction consequent upon the presence 
of an extra Y-chromosome is styled secondary non-disjunction. Two 
additional types of zygotes are produced as a result of secondary non- 
disjunction, those of the constitution YY which die, and those of the 
constitution XYY, which make up half of the males and are not ex- 
ceptional with respect to their characters but which can transmit non- 
disjunction to a certain proportion of their offspring. It will also be 
noted that of the regular daughters half are of the constitution XXY. 
They possess the power of producing exceptions on account of the 
presence of the extra Y-chromosome, but they can only be distinguished 
from their normal sisters by breeding tests or less conveniently by 
cytological examination. It is evident that an F, population such as 
this from the mating of a vermilion female to a red male is very different 
from that which is normally obtained. 
Bridges has followed out very skilfully many of the consequences of 
the assumption that these exceptional individuals are actually due to 
non-disjunction of the sex-chromosomes and consequent production of 
various types of abnormal chromosome constitution. Thus if we con- 
sider the exceptions produced by a non-disjunctional female, it is clear 
that they are a consequence of heterosynapsis in the female. Now 
when the X-chromosome pairs with a Y-chromosome in synapsis, it 
very evidently has no opportunity to exchange chromatin material 
with the free X-chromosome. Accordingly all the XX eggs and con- 
sequently all the exceptional daughters from such a female will belong 
to non-cross-over classes. A consideration of an actual experiment 
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