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 Zodlogical 
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- 
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