6' 

 A NEW TYPE OF HEEEDITAEY BEACHYPHALAGM 



IN MAN. 



I. INTRODUCTION. 



No other field in biology has within recent years yielded such far- 

 reaching and important results as the field of genetic work. Thanks 

 to the introduction of analytical experimentation, one after another of 

 the central problems in natural science has been solved or brought 

 near to a solution — ^problems which after so many vain attempts 

 seemed almost beyond the reach of ordinary scientific analysis. The 

 progress has been so considerable that one of the most prominent 

 workers in this field, T. H. Morgan, in 1916 felt justified in stating: 

 "I venture the opinion that the problem of heredity has been solved." 



It scarcely needs to be pointed out that the broad general significance 

 of the hereditary phenomena makes it urgently necessary to determine 

 whether the results obtained through experimental work with animals 

 and plants may be applied to the inheritance of human characters. 

 There was a priori no reason to doubt that this would prove to be the 

 case, and several human characters, physiological as well as patho- 

 logical, have been shown to be inherited in a way fully accordant with 

 the laws established through experimental genetic work. 



Human material, however, presents several serious obstacles for 

 genetic analysis. Not only the principal handicap involved in the 

 lack of experiments, but also other special features make man a very 

 poor subject for work in heredity. The number of individuals within 

 each family is very small; intermarriage is comparatively rare; the 

 interval of time between the generations is very long; and the charac- 

 ters studied are rarely amenable to accurate measurements. 



Only the application of the most general hereditary laws, those of 

 Mendelian inheritance and of sex-linked inheritance, has accordingly 

 so far been possible with human material. Still, the experimental 

 work has shown that even though these laws form the basis of all our 

 knowledge of heredity, their manifestation is far more complicated 

 than it would seem from many earher investigations, where the case 

 seemed closed when it was possible to demonstrate the occurrence of 

 Mendelian segregation. 



In spite of the difficulties and the incompleteness of the data which 

 are due to the nature of this material, it is nevertheless desirable, 

 whenever human characters are observed which are accessible for 

 genetic analysis, to carry out this analysis and determine, if possible, 

 whether or not the principles estabUshed through experimental work 

 may be extended so as to include human material. 



The hereditary character which forms the subject of the following 

 investigation is a symmetrical shortness of a single (the second) 



