THE LAWS OF HEREDITY 



MENDELISM AND INSANITY 



As an illustration in point, let us note the re- 

 sults of some recent studies which give us new 

 light on the heritability of that most pitiable of 

 human afflictions, insanity. 



It seems to be established that the forms of 

 nervous instability that lay the foundation for 

 insanity tend to act as Mendelian recessives in 

 heredity. 



It follows that if an insane person is mated 

 with a perfectly normal one, the offspring will 

 probably be personally normal, although carrying 

 the factors of nervous instability in their germ- 

 plasm. But if two individuals having this heri- 

 tage are mated, even though both are personally 

 normal, there will almost certainly be evidence of 

 nervous instability in at least one in four of their 

 offspring. 



Consideration of the Mendelian formula, which 

 has been fully stated in earlier chapters of this 

 book, makes it clear why the examination of 

 pedigrees, to determine whether heredity enters 

 into the causation of any given case of insanity, 

 must be extended beyond the first generation of 

 the ancestry, and in collateral lines. 



It will be recalled that a recessive trait makes 

 itself tangibly manifest only when the factors for 

 recessiveness are combined in the germ-plasm, 

 uncomplicated by the presence of the opposite 

 factors ; and hence that the recessive traits always 

 " breed true." This explains the observation of 



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