118 EVOLUTION AND GENETICS 



may be stated. If the genes lie along the length of 

 the chromosomes and if crossing-over is as likely 

 to occur at one level as at another, then, the nearer 

 together two genes lie the less likely is a break be- 

 tween them, or conversely the further apart in the 

 chromosome they lie the more likely is crossinc^-over 

 to take place. In other words the percentage of 

 crossovers is an index of the distance apart of the 

 genes. On this basis the location of the genes, as 

 shown in figure 38, has been determined. From such 

 a chart one is enabled to calculate what the inher- 

 itance of any gene will be with respect to any other 

 gene in its group provided its relation to two other 

 genes is known. 



The theory of crossing-over enables the geneticist 

 to predict the results of a given experiment with the 

 same precision that Mendel's two laws allow predic- 

 tion for a single pair of characters in the same chro- 

 mosome pair, or for two or more pairs of characters 

 in different chromosome pairs. 



