326 AN INTRODUCTION TO MODERN GENETICS 



which demonstrates that the two conditions are not quite identical. In 

 this way genetics may be able to render valuable service to medicine 

 by indicating a more fundamental classification of diseases. It seems 

 clear, for instance, that in the psychological sphere many genetically 

 and medically different entities are at present grouped together under 

 such terms as feeblemindedness and schizophrenia. 



The second sort of complication which must be expected is in a way 

 the reciprocal of this. The same gene may have different effects in 

 different genotypes or different environments. There are two causes of 

 variability involved. On the one hand, it is quite likely that some of the 

 human genes have a penetrance less than lOO per cent. For instance, it 

 is very probable that diabetes is inherited by a recessive gene,^ but one 



Fig. 133. Blood Group inheritance. — ^The most important groups are those 

 determined by the genes A, 6, and 0, which are all allelomorphs. When bloods of 

 the different groups are mixed, the reaction between the agglutinins and agglu- 

 tinogene causes the blood corpuscles to adhere, as shown in the tables. 



tends to find a smaller proportion of diabetics than would be expected 

 from the various types of matings. The penetrance of the diabetes gene 

 is not complete; but in the homozygous recessives in which it has not 

 produced diabetes it may have a milder effect in causing abnormal 

 blood-sugar values and reduced sugar-tolerance, so that such individuals 

 can be identified by suitable tests and steps taken to control the disease.^ 

 On the other hand, the expression of a gene is often considerably 

 influenced by the external environment in which the zygote develops. 

 We must never lose sight of the fact that a certain gene in a certain 

 genotype determines only the potentiahties of the zygote, and that in 

 development only some of these potentiahtieflr^*^all be reaUzed. Genetics 

 requires a concept akin to the old idea of "diathesis." A man with "a 

 gene for a character ^" has in reaUty a capacity, determined by the 

 whole genotype as well as the particular A gene, to develop the charac- 

 ter A under certain external conditions. If the conditions are not 



^ Pincus and White 1934. 



2 For a review of hereditary metabolic disorders in man, see Macklin 1933, 

 1934. 



