GENETIC SYSTEM: RELATION TO CHARACTERISTICS lOI 



develop. All the daughters live, since they all receive a 

 normal X-chromosome from the father. The consequence 

 Is that in such families there are twice as many daughters as 

 there are sons. In families of lOO to 200, as occur in 

 Drosophila, this is very striking. 



X-chromosomes are known to play in other organisms 

 roles similar to those mentioned above for Drosophila, 

 although in no other organism has the matter been so fully 

 studied. 



We may summarize what has been brought out above 

 In the following statements: 



1. Many defects that appear In Individuals are the re- 

 sult of defects in the X-chromosomes that they bear. 



2. Some of these defects are dominant; they are mani- 

 fested in all individuals in which the defective X is present. 



3. But most such defects are recessive; they are mani- 

 fested only in individuals in which the defective X is the 

 only kind present. 



4. When both a normal X and a defective X are present, 

 in most cases the normal X performs the required functions, 

 so that the individual Is not defective. 



5. Since the female has two X-chromosomes, while the 

 male has but one, such defects are more frequently mani- 

 fested in the males. Usually one of the X's present in the 

 female Is without the defect, so that she is not defective. 

 But when the male carries a defective X-chromosome, the 

 defect is manifested. 



6. It Is thus advantageous, so far as the occurrence of 

 defects is concerned, to have two X-chromosomes rather 

 than one. 



7. This gives the female a considerable advantage over 

 the male in these respects. There are many different types of 

 defects due to defective X-chromosomes; almost all of them 

 are more common In males than In females. Some of these 

 defects seriously injure the health, or even result In death. 



