THE INHERITANCE OF FAMILY TRAITS 133 



normal parents usually produce only normal offspring. In 

 case the single known parent is normal and has affected off- 

 spring it is presumed that the imknown spouse was affected. 

 On the whole, epidermolysis seems to be due to the presence 

 of a distinct factor, absence of which results in normaUty. 



The eugenic teaching is then that two normals belonging 

 to such a family as that of Fig. 105 may marry with impunity 



I 



o3 liO 



i L SiAi 



Fig. 105. — Pedigree of a family showing epidermolysis bullosa, behaving 

 like a dominant trait — appearing in each generation. Only in two instances, 

 at the right of the chart, does a case arise from a parent not known to have 

 the trait. Gossagb, after Bonajuti, 



but that in the case of parents who have, or had in childhood, 

 epidermolysis probably at least half of the children will be 

 similarly affected. 



b. Psoriasis (itch). — The question of the inheritability of 

 this disease has been much discussed. Some declare it is due 

 to infection, others deny it. Various experiments have been 

 tried. Schamberg (1908) performed auto-inoculation in 

 23 cases and got a positive result in only 3. Inoculation into 

 normal human subjects — ^usually the experimenter's own 

 body — ^have produced the disease in only one case (that of 

 Dr. Destot). On the other hand in about a third of the 

 cases observed by various physicians psoriasis was recog- 

 nized as a family disease. The most reasonable explana- 

 tion is that the disease is due to a parasite to which most 



