XII THE CHROMOSOME THEORY 141 



this is the phenomenon we have already described 

 as coupling, in which the F^ produces an excess of 

 the two types of gamete that went to its own 

 making — in this case grey normal and black 

 vestigial. But if the factors for grey and normal 

 wing are situated in the same chromosome, how 

 comes it that the F^ ? produces any of the other 

 two types of gamete, the grey vestigial and the 

 black normal, in which the factors for grey and 

 normal have become dissociated from one another ? 

 Why is the behaviour of the F^ 9 so different from 

 that of her brother ? The American observers have 

 sought to surmount this difficulty by an ingenious 

 hypothesis which provides for the interchange of 

 material between the paternal and maternal members 

 of the pair of chromosomes. The cells of an 

 individual contain, as we have already seen, a 

 double set of chromosomes, there being as many 

 pairs of chromosomes in each of these cells as there 

 are single chromosomes in the gamete. Of each 

 pair in the double set one is derived from the 

 paternal and the other from the maternal gamete. 

 When a cell divides to form two daughter cells each 

 individual chromosome is halved, so that in respect 

 of the number and constitution of the chromosomes 

 the daughter cells are like their mother cell and 

 like each other. In order to interpret linkage of 

 factors it is supposed that at certain stages pre- 

 ceding the formation of gametes there is a trans- 

 ference of material from one chromosome of any 

 given pair to the other. No such transference, 

 however, takes place between chromosomes belong- 

 ing to different pairs. The explanation will perhaps 



