GENETICS AND EUGENICS 



255 



these possibilities, it is evident that fertile families must provide for 

 more than replacement if the group to which they belong is to be per- 

 petuated. Various computations have been made of the average num- 

 ber of children per fertile family necessary to maintain the numerical 

 strength of a group. These estimates range from 3.1 to 4. Consider- 

 ing that the current incidence of childless families in America is 

 20 per cent, probably the higher number is more nearly correct, and 

 it is not safe to place the figure at less than 3.5. 



Family Size in Dysgenic Groups 



Various studies have shown that larger families occur among 

 people who have but a poor store of those qualities of intelligence, 

 stability, and physical traits that go to make up racial excellence. 



o 



11 



III 



6 



m 



LEGSND 



o 



Uale without trait 



Female without trail 



IT 



o o 



o 



VaIo and female with trail; 



Sex unknown; ? inheritance unlSOVIL 



Pig-. 84. — Standard pedigree chart. Charts such as this can be used in tracing 

 a trait through several generations of a family. This particular chart shows the 

 inheritance of a dominant trait starting with a cross between a male who was 

 heterozygous for the trait and a female in whom it was homozygously recessive. 



