Among higher animals, however, it is impossible to 

 bring about as close fertilization as that occurring in self 

 fertilizing plants, as none are hermaphrodite. 



Mendelism throws light on the question. It shows 

 that in-breeding in itself is not necessarily injurious, but 

 great care must be exercised to prevent injury. It is clear 

 that as organisms become more closely related they tend to 

 become homozygous, and many become practically so 

 through long in-breeding. Their characters exist in the 

 duplex or double-dose condition, as in Pure Lines. If the 

 essential characters in both parents are strong and without 

 defects the result of in-breeding will be the strengthening 

 or fixing of these same characters. The duplex dose of de- 

 terminers develops prepotency in all characters, good and 

 bad alike. The most important characters relate to vigor 

 and fertility, and the breeder must see to it that these are 

 in no way impaired by in-breeding. 



While undoubted evidences of prepotency, or the super- 

 ior power of some animals of impressing characters on their 

 offspring, are quite abundant, a full analysis of the factor 

 concerned has not yet been made. It is probable, how- 

 ever, that the analysis will be along Mendelian lines, as 

 suggested above, and that prepotency results from (1) 

 in-breeding where homozygous dominants tend to be pro- 

 duced; (2) assortative mating — where like mates with like in 

 regard to certain characters; (3) the chance presence of a 

 large number of dominant traits in one individual; (4) sex- 

 linkage and crossing over; (5) the action of multiple allelo- 

 morphs; or (6) variations and interrelations of the factors 

 concerned. 



If, however, some of the essential characters are weak 

 in the parents the progeny of in-breeding will show this 

 weakness intensified. 



In-and-in-breeding has developed the greatest breeds 

 of horses, cattle, swine and sheep. It is a fact that the 

 race-horse is more nervous and more delicate of constitu- 

 tion than the draft-horse, but in-breeding did not produce 

 these apparently undesirable qualities. The quality of 

 nervousness has been selected as essential to a good race 

 horse. 



In nature there are vigorous self-fertilizing and cross- 

 fertilizing flowering plants, but the latter preponderate. 



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