1 50 GENETICS 



somes, when exchange occurs, the following facts are 

 known : 



A. The exchange of parts occurs while the chromosomes 

 are conjugating in the process of forming germ cells. 



B. They are then in the form of long threads, with cor- 

 responding thickenings on the two threads (figure 37). 

 These thickenings probably show the location of genes. 



C. These threads become closely united; they are seen 

 under the microscope to be intimately intertwined. 



D. Later they separate, and study of the characteristics 

 of the individuals produced from the germ cells, as de- 

 scribed earlier, shows that at a certain point the original 

 threads have broken, so that part of the thread p has be- 

 come united with the remainder of thread m, as shown at 

 B in figure 34. Thus a consecutive series of genes from 

 chromosome p has become united with a similar series from 

 chromosome m, as shown in the figure. 



Thus far, all the relations we have examined agree with 

 the idea that the genes are arranged in the chomosome in 

 a consecutive series, and that crossing-over occurs by break 

 and exchange of parts in two such threads. 



Certain relations brought out on earlier pages furnish 

 further tests of this theory. 



4. On page 132 it was seen that when two chromosomes 

 that are defective in different ways are united in the same 

 cell, by mating parents bearing the two kinds of defects, 

 the offspring produced are usually dominant normal. The 

 reason suggested was that the two recessive defects are in 

 different parts of the two chromosomes (figure 31, A). 

 Therefore each chromosome supplies a dominant normal 

 part or gene corresponding to the recessive gene of the 

 other, so that the individual is normal. In view of the 

 known fact that there are many genes in the chromosome, 

 these relations can be illustrated as in figure 35. If a and k 

 are the two defective genes, chromosome 2 supplies a domi- 



