208 GENETICS 



The whole matter of inbreeding and the part it plays 

 in emphasizing defects has received a fresh interpre- 

 tation in the light of Mendelism. 



There is a widespread popular belief that inbreeding 

 is injurious and that it is necessary to outcross in 

 order to maintain the vigor and avoid the defects of 

 any line, but inbreeding in itself may not necessarily 

 be injurious. The consequence of inbreeding as shown 

 by the working of Mendelian laws is that latent or 

 recessive characters tend to become homozygous and 

 so brought to the surface, while outcrossing brings 

 about the formation of heterozygous traits which mask 

 recessive characters and render them ineffective. 



In the case of mankind, consanguineous marriage 

 of various degrees has long been forbidden by law or 

 custom in many races, particularly among the Jews, 

 Mohammedans, Indians and Romans. On the other 

 hand, the Persians, Greeks, Phoenicians and Arabs have 

 freely practised inbreeding, while one of the longest 

 of known human pedigrees, a royal line of Egypt, is 

 notorious for close inbreeding, even to the mating of 

 brother and sister. 



There has been a greater degree of inbreeding in 

 the Puritan stock of New England than is commonly 

 realized. David Starr Jordan points out that a child 

 of to-day, supposing no inbreeding of relatives had 

 occurred, would have had in the time of William the 

 Conqueror, thirty generations ago, 8,598,094,592 

 living ancestors. If this theoretical supposition were 

 really so, it would seem quite possible for every New 

 Englander to-day to have at least one ancestral 

 representative who won glory under William. 



