THE GREAT RELAY RACE 46i 



hybrid combination together, stayed together in linkage, while in the 

 remaining 301 cases out of 8368, crossing-over occurred between the 

 colored (C) and the starchy (*S) genes derived originally from one 

 grandparent, with the corresponding colorless (c) and wrinkled 

 (s) genes furnished by the other grandparent. These cross-overs were 

 a new combination in corn, namely, colored-wrinkled (Cs), and 

 colorless-starchy (cS). 



Chromosome Maps 



By experiment, particularly with Drosophila, which lends itself 

 especially to this kind of investigation, varying percentages of crossing- 

 over between different pairs of genes located in the same pair of chro- 

 mosomes have been determined. This method of taking advantage 

 of the occurrence of crossing-over has led to the determination of the 

 distance hetween individual genes in particular chromosomes, depending 

 upon the principle that the nearer together two pairs of genes are, 

 the more likely they are to remain linked when the chromosomes twist 

 about one another and subsequently break and rejoin, while the 

 farther apart they are, the more likely they are to shift from one 

 chromosome to the other during synapsis. 



For example, if the percentage of crossing-over, as shown by the 

 results of breeding, between the hypothetical genes Aa and Bh, is five, 

 and that between Bh and Cc is twenty, then the cross-overs between 

 Aa and Cc ought to be twenty-five (5 + 20) if the order of the genes in 

 the chromosomes is A-B-C, or fifteen (20 - 5) if the order of arrange- 



15 /^-5^B 



20- 



I I I I I 

 -15 ^^ 25 



The determination of the order of genes on a chromosome. 



ment is C-A-B. This kind of confirmation has been repeatedly 

 verified in actual breeding experiments. 



By an extension of this technique it has been possible to construct 

 chromosome maps, in which the location of the different invisible genes 

 in the various chromosomes can be determined with astonishing 

 accuracy. Such a map of the four different chromosomes in Droso- 

 phila, as far as it had been completed in 1926. when Morgan published 

 "The Theory of the Gene," is shown on page 462. Today the chro- 

 mosome map of Drosophila, like a recent map of the world as com- 



