26 GENETICS 



The bar-eyes have thus gone along with the father's X. 

 One is led therefore to suspect that the father's X-chromo- 

 some may be abnormal, and that this is what produces the 

 abnormal eyes. If this is the case, then in later generations 

 any individual that gets an X-chromosome descended from 

 that of the father will have bar-eyes. 



Is this true? It may be tested further, as follows: The 

 daughters produced in the experiment just described have 

 one X-chromosome from the original father (white), one 

 from the original mother (black). Mate these to their 

 brothers, that have only an X-chromosome from the mother 

 (black). 



Figure lo shows the result. These daughters give X-chro- 

 mosomes both to their sons and to their daughters. Half 

 the sons and half the daughters get a "black" X-chromo- 

 some (from the original mother), while half of them get 

 a "white" X-chromosome (from the original father). And 

 half the sons have bar-eyes, half of them normal eyes. Also 

 half the daughters have bar-eyes, half of them have normal 

 eyes. All the children, whether male or female, that get an 

 X-chromosome from the original father have bar-eyes, and 

 all that do not have normal eyes. 



This can be tested farther by breeding many successive 

 generations. This has been done; the matter has been tested 

 in literally hundreds of thousands of cases. In every case, 

 the results are such as above described. All the individuals, 

 in any generation, that get an X-chromosome from the 

 original abnormal parent, have the abnormal bar-eyes. All 

 of those that do not get such an X have normal eyes. The 

 bar-eye invariably follows the course of a particular X-chro- 

 mosome and its descendants, appearing wherever that 

 X-chromosome is present. This is a statement of positive 

 fact, it is not theory; bar-eye follows a particular chromo- 

 some wherever it goes. 



These experiments illustrate what is meant by substi- 



