GENETIC SYSTEM: RELATION TO CHARACTERISTICS 97 



in this case has the dominant characteristic (barred), and 

 all the offspring are dominant hke the father instead of like 

 the mother. When the mother is dominant ("barred") the 

 result in the fowls is criss-cross inheritance; while in the 

 cases before described, criss-cross inheritance occurs when 

 it is the father that is dominant. In fact, in the birds, the 

 males and females simply exchange roles, as compared with 

 their roles in the organisms of Group I, This appears 

 clearly from the following comparison of results in the two 

 cases: 



Parents Offspring 



(i) Group I, Dora. Mother, Rec. Father "I .,, r\a • -n • ^ 

 r- IT r» t- .u D x/r .u r ■^11 Offspring Dominant 



Group II, Dom. Father, Rec. Mother J r & 



Chromosomes — XX xo Xx — XO 



(2) Group I, Dom. Father, Rec. Mother: — Dom. Daughters, Rec. Sons 

 Group II, Dom. Mother, Rec. Father: — Dom. Sons, Rec. Daughters 

 Chromosomes — XO xx Xx — xo 



Since the roles of males and females are interchanged in 

 the two groups, their chromosomal conditions must also be 

 interchanged; that is, since in Group I the female has two 

 X's, and in Group II the male plays the same role as the 

 female of Group I, the male of Group II must have two 

 X's, the female but one. The chromosomal conditions in the 

 two groups are shown in the above tabulation, X signifying 

 dominant, while .v signifies recessive, and o signifies the lack 

 of X (whether Y is present or not). Careful examination 

 shows that this is the only way in which the results in Group 

 II can be produced. 



It was in the way illustrated above that the existence of 

 Group II was discovered. Birds and certain moths show 

 inheritance of the kind typical for Group II. In both these 

 it has since been found under the microscope that the males 

 have indeed one more chromosome than the female. 



