134 HEREDITY IN RELATION TO EUGENICS 



persons are immune; and that lack of immunity is an in- 

 heritable trait. 



Besides skin diseases due to infection there are other ab- 

 normal conditions consisting of irregularities or exaggera- 



Hn 



1 Atd A AiQ 



I other Females I 



I AIJ no rmal I 



Fig. 106. — Pedigree of ichthyosis. All affected persons are from non- 

 affected females. Bramwell, 1903, p. 77. 



tions of the process of rendering the outer layer of the skin 

 horny. The liability to these diseases is usually recognized 

 to be hereditary. 

 c. Ichthyosis or xerosis (xeroderma). — This is a dryness 

 ^1 — I of the skin in which plates are formed 



^ like the scales of a fish. The dis- 



ease is remarkable because, appar- 



heredity by sex and sometimes not, 



— in different families. At least, in 



BjO two of the pedigrees (Figs. 106, 108) 



I males only are affected and inherit- 



6X J_^ X ance is through a normal female. 

 • Q • But in other cases (Figs. 107, 109) 

 ■ Ju""- ^^'^uZ^^-^^'rl ""^ the females seem to be affected 



ichthyosis, behaving like a 



positive trait. Bbamwell, equally with the males and the pe- 

 ^^^^' culiar skin condition is transmitted 



either by normal or by affected females. Ichthyosis is es- 

 pecially apt to be found in families in which consanguineous 

 marriages occur and this fact, together with the pedigrees, 



