366 



EVOLUTION AND GENETICS 



Inheritance of Defects. The lesson taught by these cases of 

 human inheritance is rather obvious. Many defects are heritable 

 which are not necessarily serious but we must recognize the 

 occurrence of other heritable defects which are not only a serious 

 handicap to the individual but a menace to society. Like the 

 simple morphological defects these may appear or be concealed 

 in an individual; if the carrier finds a normal mate, the latent 

 defect need never appear in his descendants, but if, as so often 

 occurs, similar individuals mate, there is little hope that they will 

 produce normal offspring. Their appearance depends, of course, 

 on the method of inheritance. If dominant a defect must appear 

 more often than if recessive. The following table indicates the 

 expectation for all possible crosses. 



The Mendelian Expectation for Defects 

 (After Walter) 



Individuals with determiners for dominant defects cannot be 

 unaware of their defectiveness. There is a possibility that defective 

 parentage may produce normal offspring in some cases, but it is 

 obviously much more certain if only one parent shows the defect. 

 When the defect is recessive it is impossil)le to know of its presence 

 in a carrier except through his ancestry or his offspring. If the 

 defect is evident the individual is certainly a homozygous recessive 

 and unless mated with a normal individual some of his offspring 

 are certain to be defective. 



Human Pedigrees. The heritability of serious defects, such 

 as insanity, epilepsy, cretinism, pauperism and the socially less 



