THE INHERITANCE OF FAMILY TRAITS 135 



suggests that it is due to the absence of some factor that con- 

 trols the process of cornification of the skin. On this hypoth- 

 esis a normal person who belongs to an afifected family 





Fig. 108.— Pedigree of a family with ichthyosis. Note that only males are 



affected. Bond, 190.5. 



may marry into a normal family with impunity, but cousin 

 marriages are to be avoided. 



d. Thickening of the outer layer of the skin is a disease 

 that is closely related to the foregoing. In the generaUzed 



OM 



OrnnaooJ 



JiTO^ 



Fig. 109. — Pedigree of a family showing general ichthyosis, giving evidence 

 that it is a positive trait. Gossage, 1907, p. 342. 



forms (called hyperkeratosis) infection has been alleged as 

 a cause; but if infection plays a part it seems to be effective 

 only where there is a susceptibility. Evidence for contagion 

 is said to be given by the case where the only two affected 

 children were those who, alone, were nursed by their mother, 

 an affected woman. But, on the other hand, the fact that 

 the mother had the disease proves her susceptibihty. 



