PHENOMENA OF INHERITANCE 435 



be color-blind. Consequently we have the curious phenomenon of 

 simplex color-blindness appearing only in males and being transmitted 

 to them only through apparently normal females. 



E ties ( 'h rovwsomcs 



<5> X . Parents 



d\ /? cJ ? 



x 6) ' 



Gametes 

 ? i 



2T : 2YS Fi 



x 9) 



U © <3 



Gametes 



€> & <3> «> H XI 10 X0 f* 



? ? c? tf 9 ? c$ c5 



Fig. 63. Diagram of Inhekitajicb of Color-blindness through tut. Male 

 A color-blind male (here black i transmits his defect to his grandsons only. The cor- 

 responding distribution of the sex chromosomes is shown on the right, the one carry- 

 ing the factor for color-blindness being black. (After Morgan.) 



On the other hand, if a female is color-blind she has inherited it from 

 both father and mother, i. c, the character in her is duplex, and in all 

 of her children by a normal male the character will be simplex: accord- 

 ingly, all of her sons will be color-blind and all of her daughters will be 

 normal, though carrying the simplex determiner for color-blindness. 



In all cases dominance means merely the development in offspring 

 of certain characters of one parent, while contrasting characters of the 

 other parent remain undeveloped. The appearance of any developed 

 character in an organism depends upon many complicated reactions of 

 germinal units to one another and to the environment. Under certain 

 conditions of the germ or of the environment some characters may 

 develop in hybrid-; to the exclusion of their opposites, whereas under 

 other condition- these results may be reversed or the characters may be 

 intermediate. The principle of dominance is not a fundamental part 

 of Mendelian inheritance. Even when the characters of hybrids are 

 intermediate between those of their parents, if the parental types re- 

 appear in the F 2 generation we may be certain that we are dealing with 

 cases of Mendelian inheritance. 



3. The Principle of Segregation. — The individuality of inheritance 

 units, and their segregation or separation in the sex cells and recombina- 

 tion in the zygote are fundamental principles of the Mendelian doctrine. 



