ARCHITECTURE OF THE GERMPLASM 235 



tion. This hanging together of neighboring genes of 

 the same chromosome throughout the complicated pro- 

 cess of meiosis is termed linkage. 



It is extremely fortunate for the evolution of our 

 knowledge of the mechanism of heredity that Mendel 

 happened to work upon characters located in separate 

 chromosomes and so was able to establish the law of 

 the independent segregation of unit characters before the 

 apparent contradiction, that is, linkage, became known. 

 If he had come upon the confusing phenomenon of linkage 

 first, the discovery of the laws of Mendelism, in all prob- 

 ability, would have been long delayed. 



Bateson and Punnett called attention to linkage as 

 early as 1906 under the name of "coupling" in the case 

 of certain characters of sweet peas. A vague general 

 knowledge of many groups of correlations, such as 

 deafness and defective teeth going along with blue 

 eyes and albinism in cats, had for a long time existed. 



In Drosophila, the brilliant and extensive investiga- 

 tions of Morgan and his co-workers have resulted in 

 definitely placing something like two hundred characters 

 in four linkage groups corresponding to four pairs of 

 chromosomes. The limitation of linkage groups to the 

 number of chromosome pairs found in the organism 

 is proving to be one of the fundamental principles of 

 heredity. 



Moreover, it has been shown by reciprocal crosses 

 that linkage when it occurs is not due to some relation 

 per se between the genes but simply to the fact that 

 the linked genes chance to lie together in the same chro- 

 mosome. In other words, if two characters enter a 



