106 HEREDITY [ch. 



the fingers known as 'brachydactyly,' in 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 Nettleship 

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

 cannot see in dull light, and it behaves as a Mendelian 

 dominant, probably, however, with some complication, 

 since the numbers aflected are less than the theore- 

 tical 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 ofi*spring, a condition which is shortly 

 summarised as 'once free, always free.' 



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-afiiected individuals may have affected ofi- 

 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 



