232 HEREDITARY TRANSMISSION [CH. 



there is an unpaired chromosome in the female, with the 

 result that in the maturation of the eggs it is left in the egg- 

 nucleus of about half the eggs, and goes into the polar body 

 in the other half. If this odd chromosome is a sex-deter- 

 miner, its behaviour corresponds with the transmission of 

 the type as contrasted with the lacticolor character. It 

 should be mentioned, however, that there are peculiarities 

 in regard to this chromosome which have hitherto received 

 no satisfactory explanation 1 . 



This correspondence between the transmission of charac- 

 ters showing sex-limited (sex-linked) inheritance and of the 

 sex-chromosome is so exact that it is hardly possible to 

 doubt the existence of a causal relation between them, but 

 the evidence has been still further strengthened by the work 

 of BRIDGES in MORGAN'S laboratory. In most species in 

 which sex-limited inheritance is known occasional exceptions 

 occur, in which the male transmits a sex-linked character 

 to a son, or (in Lepidoptera and Birds) in which a female 

 transmits it to a daughter. Such exceptions have repeatedly 

 occurred in experiments with Drosophila, and BRIDGES first 

 suggested that they might be due to "non-disjunction" 

 of the sex-chromosomes, and then by cytological investiga- 

 tion showed that his hypothesis was correct. He suggested, 

 for example, that if a red-eyed male, mated to a white-eyed 

 female, produced a red-eyed son or a white-eyed daughter, 

 this might be due to abnormal behaviour of the sex-chro- 

 mosomes in the polar divisions of the egg, by which the two 

 ^-chromosomes after pairing failed to separate, and either 

 both remain in the mature egg or are both extruded with 

 the polar bodies. If both Jf-chromosomes (bearing the white- 

 eye factor) remain in the egg and the egg is fertilised by 

 a ^-bearing spermatozoon, the resulting zygote, of consti- 



1 DONCASTER, 1914, 19156. Cf. note, p. 171. 



