94 GENETICS 



Dominance and Recessiveness in Sex-Linked Characters 



As we have seen, and shall see further, many defective 

 conditions are due to defects in certain X-chromosomes. 

 These follow from generation to generation the distribu- 

 tion of these X-chromosomes. Most of these bodily defects 

 are manifested only in individuals in which the defective 

 X-chromosomes are the only kind present; that is, these de- 

 fects are recessive. 



But it is important to observe that in such cases the 

 normal condition of the organism likewise follows the dis- 

 tribution of certain X-chromosomes. Haemophilia follows 

 certain X-chromosomes. But in the same matings in which 

 this occurs, the healthy condition of the blood follows cer- 

 tain other X-chromosomes. If a defective mother is mated 

 with a normal father, the sons are defective because they 

 receive only the mother's defective X-chromosomes. But in 

 the same way the daughters are normal because they re- 

 ceive the father's normal X-chromosomes. In the normal 

 individuals the X-chromosome plays a part, just as it does 

 in the defective individuals. In the normal individuals it is 

 healthy and supplies what is required for normal develop- 

 ment, while in the defective individuals it fails to supply 

 what is required. 



In the case of most defects this normal condition is domi- 

 nant over the recessive condition; that is, when both are 

 present, the normal chromosome is the one that prevails in 

 its effect on the individual. There are defective characters 

 however in which the defective condition is dominant or 

 partly dominant. In these cases, when a defective X and a 

 normal one are present together, the defective one produces 

 its effect, as in the case of bar-eye, described in Chapter I. 

 In many such cases the defect is less marked when a normal 

 chromosome is present as well as a defective one; then the 

 defective condition is said to be partly dominant. 



