CONSTITUTION OF THE CHROMOSOMES 1 43 



what complex, although entirely clear In Its bearings on the 

 questions, once It Is completely mastered. It will be possible 

 to present only some of the main features of the evidence. 

 If this is followed with care It will give an understanding 

 of some of the essential features of the materials of hered- 

 ity that can be obtained In no other way. The relations to 

 be brought out are fundamental for a correct understanding 

 of heredity; they should on no account be neglected by the 

 student. 



Go back first to the experiment Illustrated in figure 32. 

 In this experiment, a "red-gray" X-chromosome from the 

 mother (A) and a "white-yellow" X-chromosome from the 

 father (B) were brought together in the cells of the daugh- 

 ters (C). There, as is shown by the nature of the sons 

 (E, F, G, H) produced by these daughters, In a small pro- 

 portion of the cells the two chromosomes exchange parts, 

 so that sons with four different types of X-chromosomes 

 are produced, In place of the two original types that were 

 present in the cells of their mothers (C). The proportion 

 of the cells in which exchange has occurred Is directly shown 

 by the proportion of the sons that have the new combina- 

 tions, red-yellow and white-gray. When this experiment is 

 performed with thousands of individuals (as has often been 

 done), the percentage of the different combinations among 

 the sons Is found to be approximately the following: 



Red-Gray 49-4 



White-Yellow 49.4 



Red-Yellow 0.6 



White-Gray 0.6 



The new combinations are the Red-Yellow and the 

 White-Gray, and together they form about 1.2 to 1.5 per 

 cent of all. That Is, exchange or crossing-over has oc- 

 curred In about 1.5 per cent of the cells of the mothers (C), 

 while In the other 98.5 per cent it has not occurred. 



