INHERITANCE OF MENTAL DEFECTS AND DISEASE 39 



near relatives may establish a certain presumption in favor of his 

 being heterozygous, but it does not prove it. 



Most of the facts of the inheritance of mental defect are con- 

 formable to the hypothesis that such defect is dependent upon a 

 number of factors instead of a single one. If the factors for 

 heritable qualities are borne by chromosomes, as there is now such 

 strong evidence for believing, is not every chromosome, or even 

 every part of a chromosome the bearer of factors that influence 

 mentality? Is it conceivable that there is a unit factor for mind 

 located somewhere in a chromosome? There may be specialized 

 parts of the chromosome complex whose influence on the develop- 

 ment of the body is such that if they are modified they produce 

 a heritable mental defect. It is of course possible that a change 

 even in a small part of a chromosome would produce the defect in 

 question. It is also possible that the development of superior 

 ability may require the influence of a special part of an indi\adual 

 chromosome. But, since in the absence of both these chromosome 

 regions we have mentioned, some type of mentality would doubt- 

 less be produced if we should get an organism at all, it seems 

 improbable a priori that the inheritance of general mental develop- 

 ment would follow the simple Mendelian formula for the inheri- 

 tance of two contrasted characters. In general, it may be prob- 

 able that the lower types of mentality are recessive to the higher 

 types much as lighter shades of coat color in mammals are usually 

 recessive (or hypostatic) to the darker shades. While a feeble- 

 minded person may be one whose infirmity is due to a particular 

 modified factor he, or at least some feeble-minded persons, may 

 owe the defect to more widespread damage to the germ plasm. I 

 very much doubt if the facts concerning the inheritance of defect 

 are as yet known with sufficient precision to warrant our tr^^ing to 

 force them into simple Mendelian formulae. Of course, if two 

 stocks differ by a single factor only, their progeny would be ex- 

 pected to afford an illustration of simple Mendelian inheritance. 

 But since the inheritance of any human family probably differs in 

 very numerous ways from that of any other, and since any change 

 in any part of the germ plasm could scarcely help having a certain 



