596 



COLLEGE ZOOLOGY 



Are diseases inherited? 



Many geneticists have attempted to deter- 

 mine the relative roles of heredity and 

 environment in relation to disease. On the 

 basis of cause, diseases are of two kinds: 

 (1) those that are primarily the result of 

 hereditary abnormal structure and function 

 of the organism and (2) those caused by 

 parasites. To the first group belongs phenyl- 

 ketonuria, a metabolic defect which results 

 in feeble-mindedness; to the second group 

 belong the infectious diseases such as tuber- 

 culosis. Infectious diseases may be avoided 

 if proper hygienic measures are adopted. 

 An infectious disease is not hereditary in 

 the sense that noninfectious diseases are. 

 Actually, what may be inherited is a tend- 

 ency or susceptibility to infectious disease. 

 In other words, people differ in their in- 

 herent resistance to certain infections. Nu- 

 merous experiments with experimental 

 animals have shown conclusively that resist- 

 ance and susceptibility to infectious diseases 

 are hereditary. The mode of inheritance of 

 some hereditary diseases is given in the list of 

 human traits, Fig. 421. 



Twins 



Twins occur more frequently than most 

 people realize; about once in every 85 hu- 

 man births, two are born at the same time. 

 Twins are of peculiar genetic interest. There 

 are two types, fraternal twins and identical 

 twins (Fig. 422). Fraternal twins develop 

 from two separate zygotes, and have, there- 

 fore, different genetic constitutions; they 

 differ both genotypically and phenotypically, 

 as do other progeny of the same parents. 

 Identical twins, on the other hand, arise 

 from a single fertilized egg, which splits 

 into two at some time during embryonic 

 development. Identical twins have the same 

 genotypic constitution, since they arise from 

 one egg. They are always of the same sex, 

 and their characteristics, both physically and 

 mentally, are remarkably similar. Any differ- 



Figure 421. Table of some human traits and 

 their usual mode of inheritance. Key to the symbols: 

 • = inheritance of trait; R-P = reduced pene- 

 trance; S-L = sex-linked; M-A = several alternative 

 genes; if more than one mode of inheritance is given 

 for a trait, it means that in some families it is in- 

 herited in one way and in others in another. 



* This is an artificial classification, because these 

 diseases actually fall into structural or physiologic 

 categories. 



