INHERITANCE OF SEX AND RELATED PHENOMENA 201 



F-chromosome pairs with an X-chromosome, the free X-chromosome 

 then goes as often to one pole as the other and this results in the pro- 

 duction of equal numbers of X, XX, XY, 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, i.e., pairing of the two Jf-chromosomes, takes place in 

 84 per cent, of cases in non-disjunctional females and heterosynapsis, 

 pairing of an X- with a F-chromosome, in 16 per cent, of cases. A non- 

 disjunctional female, therefore, will produce four types of eggs in the 

 following proportions 



4(vX)(vX) -AY:4(5(vX) :4G(vX)Y. 



When a vermilion non-disjunctional female is mated to a red male, the 

 FI 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 F-chromosome is styled secondary non-disjunction. Two 

 additional types of zygotes are produced as a result of secondary non- 

 disjunction, those of the constitution FF 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 XX Y. 

 They possess the power of producing exceptions on account of the 

 presence of the extra F-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 FI 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 F-chromosome in synapsis, it 

 very evidently has no opportunity to exchange chromatin material 

 with the free Jf -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 



