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 tbese abnormalities are 

 extremely rare and for various reasons are not likely to 

 increase. Among them may be mentioned pigmentary de- 

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

 derma pigmentosum. 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 down as largely 

 hereditary, and probably transmitted as siiigle 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- 



