MAN 231 



Does any one desire the establishment of sub-races 

 thus characterized? 



Other undesirable traits are more certainly recessive 

 and the heterozygous carriers of the factors which control 

 them cannot be distinguished by any differentiating char- 

 acters of their own. Some of these abnormalities are 

 extremely rare and for various reasons are not likely to 

 increase. Among them may be mentioned pigmentary de- 

 generation of the retina, Friedrich's ataxia, and xero- 

 derma pigTuentosum. But there are others which well may 

 give some cause for dismal forebodings — hereditary 

 feeble-mindedness and some forms of epilepsy and in- 

 sanity. These characters may be put do^vn as largely 

 hereditary, and probably transmitted as single Mendelian 

 units, but it must not be supposed that each manifestation 

 of them is of similar kind. From the graduated character 

 of feeble-mindedness and from the frequency with which 

 epilepsy and other forms of neurosis appear in feeble- 

 minded families, it is reasonable to suppose that minor 

 factors of several types play a part. Nevertheless, for the 

 deductions we wish to make here, they may be accepted as 

 true examples of Mendelian recessiveness. 



Other characters are not so simple in their inheritance. 

 The Davenports^^'^^'^^'^^ have collected a large amount 

 of data on the inheritance of skin color in negro-white 

 crosses, the inheritance of hair color in Caucasian mix- 

 tures, and the inheritance of normal differences in stature. 

 These characters are all complex. They are transmitted 

 just as are the differences in height in plants — more or 

 less of a blend in the first hybrid generation, and the 

 appearance of such second generation types as would be 

 expected if the differences were controlled by the segre- 



