BIOLOGY IN HUMAN AFFAIRS 



the pigs in the litter from this double mating were pure- 

 bred and four were crossed, as could be easily told by their 

 color. All were born on the same day, nurtured by the same 

 mother, and fed the entire time on exactly the same 

 rations. The crossbred pigs weighed 3.75 pounds each at 

 birth, the purebreds 3.23. After six months, the only two fe- 

 males of the six purebreds that were alive weighed together 

 371 pounds, while the two crossbred females weighed 456 

 pounds, making a total difference for the two of 85 pounds. 

 The two crossbred males weighed 485 pounds. 



Because crossbreds do so well, there is always the danger 

 that farmers will use these fine-appearing animals for 

 breeding. If this were generally practiced, the pure breeds 

 would soon disappear as such. The reasons why crossbred 

 animals are not desirable for continued reproduction should 

 be thoroughly understood. The dominance of favorable 

 growth factors is in the first generation, where the parental 

 races are distinct but uniform in their genetic constitution. 

 In later generations Mendelian recombination permits the 

 reappearance of weaknesses and undesirable qualities that 

 are overcome by crossing. Many of the progeny of a 

 crossbred wool and mutton sheep will have the inferior 

 fibre and light clip of one parental race together with the 

 lean body and bony carcass of the other. The breeding 

 together of Berkshire and Tamworth or Poland-China and 

 Chester White crossbreds brings forth four-legged creatures 

 that are inferior in size and vigor to the poorest specimens 

 in either purebred. 



"Registered livestock has been rigidly selected towards 

 definite standards. Each breed has certain qualities which 

 make it valued above all others. These qualities are 

 built up to a higher degree than those of any large number 

 of animals resulting from any other system of breeding. 

 The pedigree record insures that they will come fairly 

 true to that type. But purebreds, when compared with the 



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