110 GENETICS IN RELATION TO AGRICULTURE 



it is only when there are factor differences between the homologous chro- 

 mosomes that the operation of the mechanism can be detected and some 

 conception gained of its mode of operation. 



Linkage in Drosophila. To Morgan and his associates through 

 their investigations with mutations of Drosophila ampeloplila we owe 

 directly practically our entire conception of the linkage relations dis- 

 played by factors. No other single species has provided such a wealth 

 of data or proved so favorable for genetic investigations. This body of 

 data is still growing very rapidly and is adding new conceptions all the 

 time, but even at this time it is no exaggeration to say that the Zoological 

 Laboratories of Columbia University, like the old garden of the Konigs- 

 kloster at Briinn, have yielded results which will be accounted among 

 the epochal advances in genetics. Mendel's work showed that the char- 

 acters of the organism were dissociable elements of its makeup which 

 could be recombined and shuffled about in genetic experiments. From 

 this starting point the factor conception of heredity, which assumes that 

 characters of the individual may be referred to the action of definite 

 factors in the hereditary material, was developed by a host of investi- 

 gators. Morgan's work has also furnished an overwhelming body of 

 evidence supporting the factor conception of heredity, but its most im- 

 portant contribution to genetics has been in the establishment of the 

 relations existing between the factors of heredity and the chromosome 

 mechanism of the cell. 



The Four Groups of Factors in Drosophila. According to the chro- 

 mosome theory of heredity a factor is located at a particular locus in the 

 chromosome mechanism. Consequently since linkage depends upon 

 factor relations within the same chromosome it follows that the factors 

 should display linkage relations such that they would be thrown into 

 groups corresponding to the number of pairs of chromosomes. In Droso- 

 phila the linkage relations existing among over a hundred factor mu- 

 tations have been studied. The factors fall into four groups correspond- 

 ing to the four pairs of chromosomes in Drosophila, and furthermore the 

 relative sizes of these groups corresponds roughly to the relative sizes 

 of the different pairs of chromosomes. There is a large group of sex- 

 linked factors all of which display the same type of inheritance as white- 

 eye color, which has already been described. This group corresponds 

 to the X-chromosomes. There are two large groups of factors which 

 correspond to the two large pairs of autosomes, and finally there is a small 

 group, consisting as yet of only two factors, which corresponds to the 

 small pair of autosomes. The following list of the groups of factors 

 in Drosophila, although incomplete, gives some idea of the number and 

 kinds of factors which have been studied in this species (Table XX). 



The type of behavior shown in linkage in Drospohila may be illus- 



