168 GENETICS AND EUGENICS 



cases from 2 to 50 or more. However many genes there are 

 in a linkage group, each gene shows Hnkage with every other 

 gene belonging to the same group, but the apparent strength 

 of the linkage varies greatly. Under uniform environmental 

 conditions, A and B show a fairly constant 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 linkage relations, Morgan and 

 his associates have developed the chromosome 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 linkage, genes farther apart show less linkage. 



(3) Homologous chromosomes, those containing corre- 

 sponding 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 re- 

 place 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 clu-omosome. 



(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 Drosophila and their relative distances apart are shown 

 in Fig. 119. 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 and 

 the gene next to it is placed at a distance (in centimeters or 



