242 INBREEDING AND OUTBREEDING 



time. This low estimate would presuppose the survival 

 of four children per couple long enough to have their 

 mental status determined, an assumption which would 

 require a total reproductivity of six or seven children per 

 married pair. 



If, then, out of 20,000,000 pairs of married persons, 

 100,000 were heterozygous for feeble-mindedness or 

 attendant ills on both sides of the house, what would be 

 the number of such persons in the general population! 

 The problem may be stated a little more clearly : A cer- 

 tain number of persons out of a marriageable population 

 of 40,000,000 carry defective germ cells. If two of them 

 marry, one-quarter of their children will be feeble-minded. 

 If 100,000 such marriages did occur, what is the ratio of 

 these defect carriers to normals in the general population? 



Pairing among defect carriers has occurred in the 

 ratio of 1 to 200 marriages ; then these individuals musi 

 be present in the general population in the ratio of 1 to 14, 

 if no disturbing factors exist. 



The thought that one person out of every fourteen 

 carries the basis of serious mental defectiveness in over 

 half of his or her reproductive cells is enough to make the 

 stoutest heart quake. The problem of cutting off defec- 

 tive germ plasm is not the theoretically simple one of 

 preventing the multiplication of the afflicted; it is the 

 almost hopeless task of reducing the birth rate among 

 the personally unaffected transmitters where there is 



V 200 = 



approximately 13. The probability of normal mating normal = 



14 



/13\ 2 =;i69, the probability of normal mating carrier is 2 /13 _1 \ 

 \14/ 196 \14 X 14/ 



26 the probability of two carriers mating is/jA 2 = 1 . 

 196, V14/ 196 



