222 Sex-limited Diseases [ch. 



Ptosis : droop of the upper lid. 



Coloboma or h^ideremia : a congenital defect in or 

 absence of the iris. 



Ectopia lentis : dislocation of the lens. 



Glaucoma is a condition arising from various distinct 

 pathological causes. From the pedigrees I have examined 

 it is likely that in some of its forms the dominance rule is 

 followed^. 



The heredity of these various conditions could only be 

 discussed adequately at great length. To do this the time 

 is scarcely ripe. Owing to the interest now taken in the 

 problems of genetics evidence is being rapidly accumulated 

 and detailed analysis is best postponed till this new informa- 

 tion becomes available. 



Dominant Diseases or Variations in Man following a 



Sex-limited Descent. 



In discussing the phenomenon of Sex allusion was 

 made to the sex-limited diseases (p. 172). The best known 

 of these are : 



Haemophilia : the liability to bleed profusely from trifling 

 cuts or abrasions. 



Colour-blindness : in its commonest form an inability to 

 distinguish certain reds and greens. 



P setido- hyper tr op Jiic m^uscular paralysis, or Gowers 

 disease. 



Nizht-blindness of certain kinds. 



The peculiarities in the descent of these conditions are 

 that : 



(i) They affect males much more commonly than 

 females. 



(2) They may be transmitted by the affected males t, 

 but are not — save in very rare exceptions (haemophilia) — 

 transmitted by unaffected males. 



* See for instance Howe, Arch, of Ophth., xvi. p. 72. 

 t Patients sufifering from Gowers' disease rarely survive to adult life 

 and practically nothing is known of their powers of transmission. 



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