580 GENETICS IN RELATION TO AGRICULTURE 



breeding has ever been able to establish in a pure form. If more than 

 one pair of factors is concerned in such a case, the progeny is correspond- 

 ingly of greater variety- it becomes a case of the Blue Andalusian fowl 

 on a larger scale. It is probable that this condition is not often met 

 with. The only remedy for it is to change the standard of selection. 



Pedigree Breeding. If to phenotypic selection be added the concep- 

 tion of family excellence we obtain the foundation upon which pedigree 

 breeding is based. Pedigree breeding, therefore, is merely a refined 

 system of phenotypic selection; in one form or another it is a very old 

 system of breeding. The principle of pedigree breeding is a laudable 

 one, for it judges the individual not only upon its own expressed charac- 

 ters but also upon those which its ancestors have exhibited. It is, 

 therefore, one more step in the direction of strict genotypic selection. 



From an ideal standpoint the effect of pedigree breeding is to empha- 

 size the value of breeding ability. The existence of strikingly prepotent 

 animals must have been a large factor in the development of this method. 

 By insisting upon breeding ability as a measure of excellence, the tendency 

 has been to eliminate the effects of modifiability and heterozygosis, and 

 to favor the selection of the most excellent homozygous individuals for 

 breeding purposes. By so much it has concentrated blood lines within 

 breeds to a few of those which have proven most excellent, and thereby 

 it has amply justified its adoption as a method of breeding practice. 



The weakness of the method lies not so much in inherent defects as 

 in the uses to which livestock men have put it. The establishment of 

 herdbooks in which pedigrees are recorded, while undoubtedly an impor- 

 tant step in advance in the history of any breed, has tended to empha- 

 size unduly the value of pedigree, often to the extent that individual 

 excellence has not been rigidly insisted upon and even inherent family 

 defects, like the barrenness of the Bates' Duchess Shorthorns, have been 

 regarded lightly. It cannot be too strongly insisted upon that the 

 fundamental basis of pedigree breeding, as well as all other systems of 

 breeding, is individual excellence. No matter how favorable the ances- 

 try, an inferior individual within a family of superior excellence is likely 

 to have lost one or more of the factors upon which that excellence is 

 based. If that is the case, use of such an animal for breeding purposes 

 merely increases the number of animals which lack that portion of the 

 favorable genotype and by so much multiplies inferiority within the 

 family and breed. There are numerous instances in breed history of 

 pedigree fads which have worked to the ultimate disadvantage of excel- 

 lent families because of the undue prominence given to ancestry in 

 selecting breeding animals. 



Breeding Systems Based on Blood Relationship. The influence that 

 kinship has had on marriage laws in human society is familiar to all 



