CONSTITUTION OF THE CHROMOSOMES 1 55 



found, can ever be brought together into the same X- 

 chromosome; no two can ever be present in the same male, 

 though any two can be present in the same female (which 

 has two X's). 



This again of course agrees with the conception, origi- 

 nally based on other grounds, that all these are modifica- 

 tions of the same gene. When the chromosomes conju- 

 gate, the corresponding parts or genes of the two lie 

 opposite one another, and the break which results in exchange 

 of parts occurs at the same point in the two (figure 34). 

 Thus two genes of the same pair are not brought together 

 into the same chromosome. 



7. The validity of the conception that the genes are ar- 

 ranged in a chain can be tested by breaking the chromo- 

 some, through the use of radiation. This is a matter that 

 will be taken up in a later chapter; here we give only a gen- 

 eral statement of the relation of the broken chromosomes 

 to the linear arrangement of the genes. By the methods 

 already described, the order of the genes in the chromo- 

 somes is determined, so that maps may be made showing 

 this order, as illustrated in figure 38. In Drosophila it is 

 shown in this way, for example, that the genes producing 

 yellow body, white eye, abnormal abdomen and bifid wings 

 are near one end of the X-chromosome, that those for 

 vermilion eye, miniature wing and furrowed eye are near 

 the middle of the chromosome, and that those for Bar-eye, 

 cleft wings and bobbed bristles are near the opposite end 

 of the X-chromosome from that which carries the genes 

 for yellow body and white eyes. When individuals are sub- 

 jected at the proper stages of development to X-rays, the 

 chromosomes in their germ cells are frequently broken, and 

 the broken chromosomes can be detected under the micro- 

 scope in the cells of the offspring of the radiated indi- 

 viduals. Sometimes thus a small piece of one end of an 

 X-chromosome is broken off; it may then become visibly 



