158 MENDELISM CHAP. 



investigated by Dr. Drinkwater, who collected all the 

 data he was able to find among the members of a 

 large family in which it occurred. The result is the 

 pedigree shown on p. 159. It is assumed that all 

 who are recorded as having offspring were married 

 to normals. Examination of the pedigree brings 

 out the facts (i) that all affected individuals have an 

 affected parent ; (2) that none of the unaffected in- 



FIG. 33 . 



Radiograph of a brachydactylous hand. 



dividuals, though sprung from the affected, ever 

 have descendants who are affected ; and (3) that in 

 families where both affected and unaffected occur, 

 the numbers of the two classes are, on the average, 

 equal. (The sum of ^uch families in the complete 

 pedigree is thirty-nine affected and thirty-six normals.) 

 It is obvious that these are the conditions which are 

 fulfilled in a simple Mendelian case, and there is 

 nothing in this pedigree to contradict the assertion 

 that brachydactyly, whatever it may be due to, 



