96 GENETICS 



tain that the two diverse characteristics (dominant and 

 recessive) result from differences in the X-chromosomes of 

 the two parents. "Criss-cross inheritance" is particularly 

 useful in showing at once that we are dealing with sex- 

 linked inheritance. In every case where such results are 

 produced, further tests show that the two characteristics 

 follow in later generations the two different kinds of X- 

 chromosomes wherever they go. It is mainly by the use of 

 these tests that the many different characteristics dependent 

 on diversities in X-chromosomes have been discovered. 



Sex-Linked Inheritance in Group II: Sex-linked inheri- 

 tance as we have just described it was originally discovered 

 in animals belonging to Group I, in which the females have 

 two X-chromosomes, the males but one (with or without 

 a Y-chromosome). 



But when the tests we have just described were applied 

 to birds and to certain other animals, it was discovered that 

 there is a second Group (Group II) in which it is the male 

 that has two X-chromosomes, the female but one. In the 

 common fowl, reciprocal crosses were made between 

 "barred" fowls (Plymouth Rock) and black fowls (Lang- 

 shan). Here the "barred" condition was found to be domi- 

 nant. The results were: 



( 1 ) Barred father by black mother =: Barred sons -j- 

 barred daughters. 



(2) Black father by barred mother == Barred sons -f" 

 black daughters. 



Here the second mating gives "criss-cross inheritance," 

 showing that we are dealing with sex-linked characters de- 

 pendent on differences between the X-chromosomes of the 

 parents. But in the first mating above, all the offspring are 

 like the father instead of like the mother, which latter was 

 the case in the matings on the previous pages. The father 



