108 HEREDITY [CH. 



to bring out the disease may be avoided, especially if 

 it is a condition not present at birth, but appearing 

 later in life. This kind of thing may perhaps be 

 illustrated by the pedigrees of Retinitis pigmentosa 

 taken from Nettleship (Bowman Lecture, 1909 [23]) 

 on p. 105. 



The disease is not usually present at birth, but 

 comes on at a varying age, sometimes during or after 

 middle life, and it will be seen that in the first 

 pedigree it is transmitted only by affected members 

 of the family, so behaving as a typical Mendelian 

 dominant. In the second pedigree, however, it com- 

 monly 'skips' a generation, the parents of affected 

 individuals usually being normal, but themselves 

 children or sisters of those who are affected. The fact 

 that in both families (as in most cases of this disease) 

 males are more frequently affected than females, 

 suggests that there is some complication, and this 

 is perhaps connected with the fact that in one family 

 the disease behaves as a simple dominant, while in 

 the other it is most frequently transmitted, like 

 colour-blindness, through normal females from affected 

 males. These pedigrees are given as examples of 

 the somewhat irregular inheritance of diseased con- 

 ditions such as may frequently be seen in the medical 

 journals ; many of them are probably explicable in 

 the ways suggested above. 



A somewhat different group of phenomena is 



