GENETIC SYSTEM: RELATION TO CHARACTERISTICS 95 



Thus whenever an individual bearing defective X- 

 chromosomes is mated with a normal individual, we have 

 both dominant and recessive characters, the course of which 

 may be followed in the later generations — the dominant 

 character being commonly the normal or usual condition. 

 How this works out is shown in our next paragraphs. 



Tests for Sex-Linked Inheritance: By observing the dis- 

 tribution of the dominant and recessive characters among 

 the offspring of certain matings, a test for determining 

 whether given characters depend on the X-chromosomes is 

 supplied. The test consists in making what are called re- 

 ciprocal crosses. ( i ) On the one hand mate a dominant 

 female with a recessive male; (2) Also mate a recessive 

 female with a dominant male. 



Represent a dominant X-chromosome by a capital X, 

 a recessive X-chromosome by a lower-case letter x. In the 

 group of organisms with which we have been dealing, the 

 female has two X-chromosomes, the male but one. The 

 daughters receive an X from each parent, the sons an X 

 from the mother only. 



A dominant female will be represented by XX, a reces- 

 sive male by xo^ similarly a recessive female is xx, a domi- 

 nant male is XO. The two matings and their results will 

 then be represented as follows: 



(i) XX by XO gives Xat + XO 



Dom. Mother Rec. Father Dom. Daughters Dom. Sons 



(2) XX by XO gives X.x -j- xo 



Rec. Mother Dom. Father Dom. Daughters Rec. Sons 



Thus when the mother is dominant, the father recessive, 

 all the children are dominant, like the mother. When the 

 father is dominant, the mother recessive, the daughters are 

 dominant like the father, the sons recessive like the mother; 

 this is called "criss-cross inheritance." 



Whenever reciprocal crosses give these results, it is cer- 



