2IO Human Heredity [ch. 



Dominant Hereditary Diseases and Malformations 



Though knowledge of genetics of normal characters in 

 man has advanced so little, we have now clear evidence as 

 to the laws of descent followed by many striking peculiarities 

 which are of the nature of deformity or disease. It is 

 somewhat singular that nearly all the abnormal features 

 (except those which are sex-limited) that have been yet 

 positively shown to follow Mendelian rules in man are 

 dominant to the normal. There are indications that 

 certain abnormal conditions are recessive, but in two of 

 these only is there much evidence. Dominants are of course 

 much easier to trace, as the peculiarity then descends 

 directly from parent to offspring, and so a continuous 

 history is provided. Probably it is to this circumstance 

 that the comparative plenty of evidence respecting the 

 dominants is due. 



Brachydactyly, 



A good example of these dominants, which indeed was 

 the first Mendelian case to be demonstrated in man, is that 

 described by Farabee. The peculiarity consists in a short- 

 ening of the fingers and toes, which had only one phalangeal 

 articulation like the thumbs instead of two (Figs. 23 and 

 25). The condition is said to have been the same in all 

 the fingers and toes of the affected members of the family. 

 The descent is represented in the diagram. As there indi- 

 cated, the peculiarity descended solely through the aftected, 

 and the children of the unaffected did not in any single 

 instance reproduce it, for they were evidently pure reces- 

 sives. The offspring of the affected, on the contrary, 

 consist of affected and unaffected, in approximately equal 

 numbers (36 : 33). The affected parents in each case 

 married with normal persons, so these unions are all of the 

 form DR X RR, and the equality of affected and unaffected 

 is in accordance with Mendelian expectation. 



Farabee's families are American, being centred in Penn- 

 sylvania, and the diagram shows records referring to five 

 generations. Recently Drinkwater has published a full and 



