174 MENDELISM . chap. 



cestry. Once free they are always free, and can marry 

 other normals with full confidence that none of their 

 children will show the deformity. 



The evidence available from pedigrees has revealed 

 the simplest form of Mendelian inheritance in several 

 human defects and diseases, among which may be men- 

 tioned presenile cataract of the eyes, an abnormal form 

 of skin thickening in the palms of the hands and soles 

 of the feet, known as tylosis, and epidermolysis bullosa, 

 a disease in which the skin rises up into numerous burst- 

 ing blisters. 



Amorig the most interesting of all human pedigrees is 

 one recently built up by Mr. Nettleship from the records 

 of a night-blind family living near Monpelier in the south 

 of France. In night-blind people the retina is insensitive 

 to light which falls below a certain intensity, and such 

 people are consequently blind in failing dayUght or in 

 moonhght. As the Monpelier case had excited interest 

 for some time, the records are unusually complete. They 

 commence with a certain Jean Nougaret, who was born in 

 1637, and' suffered from night-blindness, and they end for 

 the present with children who are to-day but a few years 

 of age. Particulars are known of over 2000 of the de- 

 scendants of Jean Nougaret. Through ten generations 

 and nearly three centuries the affection has behaved as 

 a Mendelian dominant, and there is no sign that long- 

 continued marriage with folk of normal vision has pro- 

 duced any amelioration of the night-bHnd state. 



