INHERITANCE IN MANKIND 43 



The peculiarity descends only through the affected. 

 No unaffected person had brachydactylous children. 

 The affected parents, all of whose known offspring 

 are entered in the pedigree, married normal unaffected 

 recessive husbands or wives. Their unions are therefore 

 of the type DR = RR, when on Mendelian principles 

 the chances are that half the children will be DR, 

 and therefore will show the peculiarity, since it is a 

 dominant, and half will be RR, pure recessives in whom 

 it has been clean bred out. To bring the figure within 

 compass, the marriages of the unaffected members of 

 the family are not tabulated, but, as mentioned above, 

 in no instance did their descendants prove to be 

 brachydactylous. Of the known children of the 

 affected, it will be seen that thirty-six are brachy- 

 dactylous and thirty-three are normal — a close approach 

 to the Mendelian probability. Another family in- 

 vestigated by Drinkwater showed similar phenomena, 

 with numbers of thirty-nine to thirty-two affected and 

 unaffected children of DR = RR marriages. There is 

 little doubt that brachydactyly and its absence are in 

 man a pair of simple Mendelian factors, the malforma- 

 tion being a dominant character which appears whether 

 the Individual is a pure-bred DD or a cross-bred DR 

 — whether he gets the peculiarity from both parents or 

 from one only. 



Some other diseases, such as certain forms of cataract, 

 show signs of similar relations ; they appear to be simple 

 Mendelian dominants. No clear case of a pathological 

 condition inherited as a recessive seems yet to have been 

 traced. As such characters would not always be trans- 

 mitted directly from parent to child, evidence is much 



