GENETIC SYSTEM: RELATION TO CHARACTERISTICS 89 



the defective X-chromosome has no normal one with it, 

 all this was at once cleared up and became intelligible. This 

 is revealed at once when we place in the diagram the X- 

 chromosomes of the individuals, marking in a special way 

 the X-chromosome of the individual originally affected (see 

 figures 24 and 25), and in the same way all the X- 

 chromosomes derived from that chromosome. Every indi- 

 vidual that contains only the defective X is itself defective, 

 whether male or female. Every individual that contains in 

 addition a normal X is without the defect. Such defects 

 usually appear in males only, because males have only one 

 X-chromosome; and if that is defective, the individual is 

 defective. Females on the other hand have two X- 

 chromosomes, and as normal X's are more common than 

 defective ones, usually any female that has a defective one 

 has a normal one also, and is therefore not personally de- 

 fective. 



But rarely it happens by chance that a female gets two 

 defective X-chromosomes; then having no other, she is 

 personally defective. And in that case, since she always 

 gives one of her X's to her sons and they have no other, 

 her sons are all defective. 



Indeed, when we recognize that such a characteristic as 

 haemophilia or color-blindness is due to a defect in certain 

 X-chromosomes, but is not manifested if a normal X- 

 chromosome is present, we discover that all the results 

 enumerated above, and illustrated in the diagram of figure 

 25, are inevitable. 



Many other recessive characteristics, as we shall see, are 

 due to defects or modifications of X-chromosomes. Such 

 characters are distributed to the offspring in the way il- 

 lustrated above; they all follow the same rules of inheri- 

 tance. They follow the defective X wherever it goes, being 

 manifested whenever the defective X is the only kind of 

 X that is present. Such characters, since they are commoner 



