104 HEREDITY [ch. 



which the fingers have one joint less than the normal ; 

 congenital cataract, and probably other diseases of 

 the eye. Perhaps the most remarkable human 

 pedigree ever collected is one of 'night-blindness/ 

 extending through nine generations and going back to 

 the seventeenth century, which has been published by 

 Xettleship (see [1]). The condition is one in which 

 the patient cannot see in dull light, and it behaves 

 as a Menclelian dominant, probably, however, with 

 some complication, since the numbers affected are less 

 than the theoretical expectation. In all these cases 

 in which the abnormality is dominant, only affected 

 individuals can transmit it ; the normal members of 

 the family have only normal offspring, a condition 

 which is shortly summarised as 'once free, always 

 free.' 



But the rule that the affected alone transmits will 

 be followed only when the condition depends on a 

 single factor ; if it depends on more than one, or if 

 its dominance is modified by sex or other conditions, 

 then non-affected individuals may have affected off- 

 spring. This is possibly the case in many diseases in 

 which it appears that the affection is dominant, and 

 yet certain non-affected individuals have affected 

 offspring, and in such examples it must also be 

 remembered that the disease is probably not always 

 developed in people in whom the tendency is present ; 

 the tendency may be there but the conditions required 



