94 



second principle still applies to most 

 cases of inheritance but has been 

 modified by the finding that two or 

 more characters may not always be 

 distributed independently but when 

 entering a cross together may tend to 

 stay together, and when entering a 

 cross separately may tend to remain 

 separate (in different individuals) in 

 inheritance. This peculiarity was first 

 remarked by Bateson (1906) in the 

 case of the inheritance of flower color 

 and pollen shape in sweet peas. In his 

 experiments purple flower (as opposed 

 to red) and long pollen (as opposed to 

 round) appeared to be associated or 

 coupled in crosses so that a marked dis- 

 tortion was evident in the second gen- 

 eration ratio of 9:3:3:1 expected on 

 the usual hypothesis of independent 

 assortment, in favor of the classes 

 (purple-long and red-round) repre- 

 senting the grandparental combina- 

 tions of these characters. The opposite 

 phenomenon was noted and named re- 

 pulsion. Later, Morgan in 1910 found 

 the same phenomena while studying 

 inheritance of certain characters which 

 had arisen by mutation in the vinegar 

 fly (Drosophila melmiogaster). He 

 conceived these two exceptions to 

 Mendel's principle of independent as- 

 sortment as two aspects of a single 

 phenomenon which he termed Liyikage 

 or associated inheritance. 



The interpretation of these events 

 has constituted one of the great ad- 

 vances of biological science. Sutton, in 

 1902, suggested that the marked paral- 

 lelism between the discreteness and as- 

 sortment of unit characters and the be- 

 havior of the chromosomes might be 

 due to the residence in the chromo- 

 somes of the determinants or genes 

 representing unit characters. Immedi- 

 ately after Bateson's announcement of 

 coupling, Locke (1906) pointed out 

 the similarity between this new mode 



DUNN 



of inheritance and the results which 

 might be expected if the coupled char- 

 acters were determined in one chro- 

 mosome. The development of this hy- 

 pothesis, its proof and very important 

 extension and generalization, have 

 been the work of the American biol- 

 ogist T. H. Morgan, and of research 

 workers associated with him, assisted 

 more recently by data gathered by 

 geneticists and cytologists working on 

 many species of plants and animals. 

 There has resulted from this work the 

 elaboration of the chromosome theory 

 of heredity, for the details of which, 

 and the supporting evidence, the in- 

 terested reader must be referred to the 

 original works, especially as sum- 

 marized in two publications of Morgan 

 and his co-workers (1915 and 1919). 



For our purpose it is sufficient to 

 note in brief that the theory supposes 

 that the differential representatives of 

 heritable characters are located in the 

 nuclear material of the t^^ and sperm 

 cells, more precisely in those remark- 

 ably constant and individual organiza- 

 tions of chromatin known as chromo- 

 somes, which appear at the time of cell 

 division and which probably retain 

 their individuality even in the resting 

 stages of the nucleus. Of the evidence 

 it must be observed that the interpreta- 

 tion and proof of the theory rest en- 

 tirely on the study of linkage, or asso- 

 ciated inheritance. This phenomenon 

 is observed in the tendency which 

 characters exhibit of remaining 

 through several generations in their 

 original combinations, resulting in an 

 alteration of the expected Mendelian 

 ratios based on independent assort- 

 ment. This tendency may be absolute, 

 in which case linkage is said to be com- 

 plete. More often it is partial, that is, 

 characters originally associated may 

 separate in a certain proportion of in- 

 stances, or characters originally sepa- 



