MENDELISM 29 



mation to the 50 per cent, required by theory. Moreover, 

 you know the recessives breed true, and so we find it in 

 this family, for the normals have not in a single instance 

 transmitted this peculiar defect. Mr. Punnett refers to 

 this family as exhibiting " the most conspicuous example 

 of Mendelian heredity in man." 



More recently Mr. Nettleship, a distinguished oculist, 

 has published an account of a family affected by " congenital 

 night-blindness" (an inability to see in a dim light). It has 

 been hereditary through ten generations, and has been 

 transmitted solely by the affected individuals. If we stop 

 at the abnormal family in each line of descent, the propor- 

 tion of affected to normals agrees almost exactly with 

 Mendelian ratios. 



Here is the chart of a family affected by spasmodic 

 asthma, in which the numbers agree exactly with Mendel's 

 law : 10 individuals suffered, whilst 10 were free from the 

 disease (the descendants of the recessive must of course not 

 be counted) — 



There is a human disease called Haemophilia, charac- 

 terized by a tendency to bleed profusely from even slight 

 wounds ; as a rule the males alone suffer, and the females 

 escape, but it is quite clear that, as an unaffected mother 

 can transmit the disease to her male children, she herself 

 must possess the disease in a latent form (as an impure 

 dominant D[R] can transmit the recessive features.) 



I have already referred to the opinion that living beings 



