84 GENETICS 



pair. These relations make it a relatively simple matter to 

 discover the distinctive effects of a particular X-chro- 

 mosome. The effects of X-chromosomes will therefore 

 be dealt with somewhat fully as a type of chromosomal 

 action. Later the action of the other chromosomes will be 

 taken up. 



Method of Inheritance of Characteristics Result- 

 ing from Modifications of X-Chro7nosomes 



The X-chromosomes, as we saw in Chapter I, pass al- 

 ways from the father to his daughters, not to his sons. 

 The sons get their X-chromosomes exclusively from the 

 mother. These peculiarities make it relatively easy to dis- 

 cover characteristics of individuals that depend on the kind 

 of X-chromosomes they contain. We saw in our introduc- 

 tory account of the discovery of the materials of heredity 

 certain effects of altering X-chromosomes. Sometimes, as 

 there set forth, an X-chromosome of one of the parents is 

 defective in such a way as to cause an abnormality or defect 

 in all the individuals that receive this X-chromosome or its 

 descendants. Such an abnormality was bar-eye (figures 9 

 and 10). Characteristics of this sort, that are manifested in 

 every individual that has the defective X, are called domi- 

 nant characteristics. 



It was further set forth that there are other character- 

 istics which result from defective X-chromosomes, but which 

 are manifested exclusively in individuals that carry only 

 X's that are defective; if a normal X is present also, the 

 characteristic does not appear. Such a characteristic was 

 white eyes. Seemingly the defective X fails to produce cer- 

 tain required materials. But if a normal X is present, it 

 produces the materials, so that no defect results. Such char- 

 acteristics, that are manifested only if a normal X is not 

 present, are called recessive. 



