INHERITANCE OF SEX AND RELATED PHENOMENA 



199 



to disjoin from each other. As a result eggs are occasionally produced 

 which contain two Jf -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 JT-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 



FIG. 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)0 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 F-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 



