THE APPLICATION TO MAN 307 



That two parents born deaf do not produce more 

 than 26 per cent of deaf children is probably due to 

 the fact, first, that each parent is in all likelihood het- 

 erozygous for deafness and that, second, the same com- 

 bination of factors which is the cause of the parental 

 defect on either side of the pedigree does not happen to 

 recombine after segregation to form the new individual. 

 Deafness will be produced in the offspring only when 

 matings occur in which the proper factors are com- 

 bined. Such an undesirable result is much more likely 

 to happen if both parents come from the same, or 

 related, hereditary strains than if they are derived 

 from families in no way connected by blood. 



Herein lies the biological objection to cousin mar- 

 riage which tends to bring together, and thus to per- 

 petuate, like defects. Outcrossing, on the contrary, 

 through the law of dominance, tends to conceal defects 

 and to prevent their expression. 



If the patent parental characters were all that re- 

 appeared in the offspring, the marriage of near kin 

 would present fewer difficulties. It is the "skeleton in 

 the closet" that makes trouble. Elderton gives a case 

 of haemophilia where the direct line was free from taint 



