INHERITANCE OF SEX AND RELATED PHENOMENA 199 
to disjoin from each other. As a result eggs are occasionally produced 
which contain two X-chromosomes instead of one as is normally the case. 
In Fig. 90 are illustrated in diagram the consequences of such aberrant 
reduction divisions in the female. If the X-chromosomes fail to disjoin 
in the reduction divisions, they may be included in the egg, in which 
case an egg with two X-chromosomes is produced, or they may both be 
thrown out into the polar body, in which case an egg with no X-chromo- 
some is produced. This phenomenon Bridges calls primary non-dis- 
junction. An egg (vX)(vX) fertilized by a Y sperm gives a (vX)(vX)Y 
zygote, and it develops into an exceptional vermilion female. An O 
Fic. 90.—Diagram of the production of exceptional individuals, vermilion females and 
red males, through primary non-disjunction from matings of vermilion female by red male. 
(Adapted from Bridges.) 
egg (one which contains no sex chromosome) fertilized by a (VX) sperm 
gives a (VX)O zygote, and it develops into an exceptional red male. 
Zygotes of the constitution (_VX)(vX)(vX) and YO are, also, possible as 
a consequence of such non-disjunction but it is certain that they die, 
consequently nothing definite can be determined as to their characters. 
The proof that non-disjunction is the correct interpretation of these 
exceptional cases in the transmission of sex-linked characters has been 
established by breeding tests and by actual cytological examination of 
exceptional individuals. 
Assuming that homologous chromosomes pair in synapsis, in an XX Y 
exceptional female two types of reduction divisions are possible. If 
the two X-chromosomes pair, then in reduction they disjoin and one 
goes to each pole. The free Y-chromosome then passes as often to 
one pole as to the other, and as a consequence, two kinds of eggs, XY 
and X, are produced in equal numbers. On the other hand, when the 
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