HUMAN INHERITANCE 181 



then show the defect. Bv the time this has occurred 

 the gene may have become so widely spread that the 

 most that can be done — if something is to be done — 

 is to discourage the defective individuals that have 

 reappeared from further contamination — a council 

 of perfection that may not be appreciated. 



When the defect is dominant it will appear in half 

 the offspring^ if marriage with a normal person oc- 

 curs. The other half of the children that are normal 

 do not transmit this dominant defect, and have, so 

 to speak, escaj^ed entirely from the inheritance. 



There are, however, a number of cases where the 

 defect is not perfectly dominant. This means that its 

 variabilitv is so wide that a few individuals that 

 carry the gene may fail to show it, or show it to such 

 a small degree as to escape casual examination. 

 There are, apj)arently, cases of this sort which have 

 been utilized by the opponents of the modern the- 

 ory of heredity as justification for the statement 

 that such a character does not show "strictlv ]Men- 

 delian inheritance." Unless suitable tests are made 

 it is not possible to claim that extreme cases of this 

 sort are exceptions to Mendelian inheritance, for, 

 similar cases are known in other animals, and have 

 been shown not to be exceptions, but due to the wide 

 variability of the hybrid character. 



Aside from the major physical defects there are 

 manv smaller ones that do not interfere seriouslv 



1 Assuming the parent is heterozygous for the character. 



