LIN]!fEAN SOClEXr OF LOXDOJf. 4 1 



(4) The normals do not transmit it. 



(5) He traced the defec-t through five generations. He ex- 



pressed the opinion that the shortening was due to the 

 absence of one of the bones. The normal finger has 

 three bones ; the middle one is missing from the Bruchy- 

 dactyloLis finger. The toes also are aifected in the same 

 way. 



I have been able to make a practically complete investigation 

 of two separate families living in this country, w^hich show rhe 

 same peculiarities described by Farabee, and I can confirm all his 

 conclusions except that relating to the essential nature of the 

 abnormality. Unfortunately, Mr. Farabee drew his conclusions 

 on this point from insufficient data. 



The radiographs which I shall exhibit will speak for themselves, 

 and you will haye no diiJiculty in forming an opinion as to what 

 has happened to produce this interesting abnormality. 



Ln addition to the two families to which refei-ence has been 

 made, I have had the good fortune to meet with two others 

 showing a distinct variation, and presenting a condition inter- 

 mediate between the normal and the Brachydactylous type. It 

 is in fact a smaller degree of abnormality, and I have therefore 

 termed it Minor-Brachydactyly 



Both types afford excellent examples of Mendelian Inheritance, 

 the abnormality acting iis a Mendehan Dominant. 



An animal or plant which is dominant in the Mendelian sense 

 transmits its peculiarity to its descendants according to the 

 following scheme : — 



DD X EE 



I I 



D[E] D[E] D[E] 



When mated with an individual (ER) which lacks the character- 

 istic feature of the don)inant (DD), all the resulting offspring 

 resemble the dominant parent in ajipearance, e. g. : — 



A pure-bred tall sweet pea, crossed with a dwarf, produces tall 

 plants exclusively. Hence tallness is dominant to dwarfness. 

 The dwarf character disappears and is said to be '! recessive." 



j^evertheless the dwarf character is still there in a latent con- 

 dition, and these individuals may be represented by the letters 

 D[E]. This recessive character of dwarfness, as a matter of fact, 

 becomes manifest in the next generation, for when one of these 

 D[E] tails is crossed with a dwarf (E) the resulting offspring are 

 of both kinds, tails and dwarfs, and not only so, but the two types 

 appear in approximately equal numbers. Pure tall plants (DD) 

 produce tails only, pure dwarfs (EE) produce dwarfs only. 



