HUMAN PEDIGREES 23 



If we regard the capacity of contracting tuber- 

 culosis as due to the absence of some factor which 

 is present in normal individuals, then the parents 

 Nos. 3 and 4 and 2 and 6 may be symbolised as 

 N n, where N = normality, and n = absence of nor- 

 mality = tubercular predisposition. We shall then 

 expect in the offspring of Nos. 3 and 4, one 

 tubercular to three normal individuals. If we 

 bear in mind the fact that the young man. No. 7, 

 suffers from cough, is very slight in build, is losing 

 flesh, and looks younger than he really is, and if we 

 further remember that he has only just reached the 

 age, namely, nineteen, when the disease first manifests 

 itself in this family, there can be little doubt that the 

 prognosis in his case is a grave one. If we then regard 

 him as potentially tubercular, that will give us 3 : 3, 

 instead of the expected ratio in this particular 

 branch. 



If we make similar assumptions with regard to 

 the constitution of the two parents Nos. 5 and 6, 

 we find that in their offspring there are 4 normals to 

 3 tuberculates, instead of 5*2 to 1'8 respectively. 

 Taking the whole F^ generation together and includ- 

 ing No. 7 among the tuberculates, we have 7 normals 

 to 7 tuberculates, instead of 10*5 to 3*5 respectively. 



In regard to this discrepancy of numbers, we 

 must remember there were thirteen children who 

 were stillborn. Had they hved, it is possible the 

 ratios may have been more accordant ; they may 

 have also been more discordant. We have yet to 

 learn what is the relationship between the tubercular 



