28 MENDEL1SM 



I now come to the consideration of Mendelism in Man. 

 Here the evidence of the action of Mendelian laws is scanty, 

 but there has not been time for much investigation. The 

 best example, so far, is that supplied by a family certain 

 members of which are affected by a peculiar deformity of 

 the hands and feet — technically termed "Brachydactyly," 

 and consisting in the apparent absence of the middle bone 

 of each finger and toe. The members of this family are 

 divided into two classes : one class is quite normal, having 

 ordinary hands and feet ; the other class shows the apparent 

 absence of the middle bone in every finger and toe (not 

 reckoning the thumb and big toe). These abnormal 

 individuals are also much below the average stature of their 

 normal relatives. 



This figure shows that the middle bone is present in the 

 middle finger as a short cubical piece. It ought to be 

 intermediate in leno-th between the other two bones of the 

 finger. The cubical piece at the base of the terminal bone 

 of the other fingers is in reality the middle bone, which is 

 ill-developed, and united into one with the terminal bone. 



The middle bone of the foot is even more degenerate 

 than the middle bone of the finger, and in each case it is 

 found to form one piece with the terminal bone. 



This abnormality has been hereditary through seven 

 generations. It acts as a pure dominant. As each individual 

 has had one abnormal parent, whilst the other parent was 

 normal, they are impure dominants, or hybrids, so far as 

 this anatomical feature is concerned, and can be represented 

 by D[R]. Now when one of these abnormals marries a 

 normal, i. e. a recessive, we get D[R] x R.and you know from 

 Law 4 that one half of their children should show the 

 abnormality, and the other half should be normal. Well, 

 these abnormals have had 75 children, of whom 39 were 

 abnormal, that is, 52 per cent. — a sufficiently close approxi- 



