442 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
linkage with each other, A and C show a different and likewise fairly 
constant linkage strength, and so on through the entire group. This 
leads to the conclusion that the genes of a linkage system are bound 
together, gene with gene, with bonds of definite strength in each case. 
In order to visualize the matter and get a more objective view of link- 
age relations, Morgan and his associates have developed the chromo- 
some theory of linkage. Its essential parts are: 
1. Genes which show linkage with each other are located in the 
same pair of chromosomes. It is the substance of the chromosome 
which binds the genes to each other and causes A to be inherited 
when B is. 
2. Genes close together in the same chromosome show strong link- 
age, genes farther apart show less linkage. 
3. Homologous chromosomes, those containing corresponding sets 
of genes, one set derived from the father, one from the mother, lie side 
by side (in synapsis) previous to the formation of gametes. At this 
time breaks are likely to occur in the chromosomes and parts of one 
are likely to replace corresponding parts of the other. 
4. Such replacement is called crossing-over. 
5. Breaks are commoner in long chromosomes than in short ones, 
and between distant points than between near points on the same 
chromosome. 
6. The genes occur in a chromosome, like beads on a string, in a 
single row and in definite order. 
The supposed order of the genes in the four linkage groups of Dro- 
sophila and their relative distances apart are shown in Fig. 90 (p. 437). 
In these diagrams or “maps,”’ when the probable order of the genes 
in a system has once been determined, the supposed end gene of the 
system is placed at position o and the gene next to it is placed at a 
distance (in centimeters or other units) corresponding to the average 
cross-over percentage between the two, this process being repeated 
from gene to gene until the whole chain is plotted. The “map” is 
thus based on a summation of the distances (measured in cross-over per- 
centages) from gene to gene. But if we compare the “map distances” 
between genes not adjacent to each other in the chain with the observed 
cross-over percentages between the same genes, we find that the map 
distance is regularly greater than the cross-over percentage, except for 
very short distances (5 or less). Thus if three genes occur in the order 
ABC, it is usually found that AB+BC is greater than AC. In other 
words, the cross-over percentage between B and C is commonly 
