PRINCIPLES Of HEREDITY 



667 



between two genes is changed by crossing over, e.g., tall becomes 

 linked to pear-shaped, and then those two particular genes tend to be 

 inherited together until in some subsequent generation another crossing 

 over occurs. 



In the species whose inheritance has been studied most extensively, 

 fruit flies, corn and mice, the data on crossing over have been assem- 

 bled, and chromosome maps, showing the relative location of the 



13 



14 



15 i 



16 



17 



0.0 

 0.0 

 0.0 

 0.1 



8.1 



1.5 

 1.7 



3.0 

 4.6 

 5.5 

 6.9 

 7.5 



no 



137 

 15.0 



178 

 182 



20.0 

 21.0 



23.0 

 243 



27.5 

 27.7 



32.8 

 33.0 



36.1 

 36.2 



38.7 

 40.7 

 43.0 



18 



sp-f 66.0 



Figure 32 8 Diagram of the X chromosome of a fruit fly as seen in a cell of the 

 salivary gland together with a map of the loci of the genes located n. the >-. chromo- 

 some, with the distances between them as determined by frequency of crossu.g over. 

 (Hunter and Hunter: College Zoolog) .) 



