36 



The Journal of Heredity 



type BB X RW on this hypothesis, 

 which means that BR must be the 

 fonnula of both self blacks and self reds 

 in one cross, black-and-reds and self reds 

 in the other, while B\V must be the for- 

 mula of the roans as well as the whites. 



It remains to show that these crosses 

 can be explained by the hypothesis of 

 independent extension and intensity 

 series. The necessary assumptions are: 

 first, that there is a unit Alendelian 

 difference between Yorkshire and Tam- 

 worth in their reaction toward extended 

 black, the former possessing a dominant, 

 the latter a recessive restriction factor; 

 second, that there is a unit Mendelian 

 difference between the white of York- 

 shire (dominant dilution) and the red of 

 Tam worth. The roan York-Tamw^orth 

 hybrids are thus double heterozygotes. 

 The Hampshires and Poland Chinas 

 transmit more or less extended black 

 which ser\'es to Ijring out the difference 

 between the two restriction factors, and 

 they also transmit a condition of inten- 

 sity of red to which white of Yorkshire is 

 dominant. The cross should produce 

 blacks with more or less red, blacks with 

 more or less white, self reds, and self 

 whites in eciual numbers. In the Hamp- 

 shire cross tlie first two classes become 

 indistinguishable, both beins( self black. 

 The results are in close agreement with 

 this expectation. The production of 

 solid red offspring from a pure Hamp- 

 shire f)r Poland China sire can hardly be 

 explained exce])t as due to combination 

 of a dominant restriction factor from 

 the Yorkshire with the intensity of red of 

 Tamworths. The possiljility of produc- 

 ing a red breed dominant even over the 

 black of Hampshires is ])ractically 

 demonstrated. 



Thus restriction of Yorkshire and re- 

 striction of Tamworth show a segregat- 

 ing difference in their reaction to ex- 

 tended black, it has also been noted 

 that extension of Hami:)shire and of 

 Berkshire or Poland China show dif- 

 ferences which are increased by crosses 

 with a given red. The difference 

 betw'cen Berkshire or Poland China and 

 Tamworth or Duroc, on the other hand, 

 tends to be reduced bv crosses and there 



seems to be no good evidence of segrega- 

 tion. Berkshire extension comes out 

 of Yorkshire crosses with almost as 

 much white as black, while Duroc 

 restriction often comes out of York- 

 shire crosses w4th black spots. Thus it 

 is not impossible that Berkshire black 

 and Duroc red may possess the same 

 main factors for extension but have 

 become distinct in ajipearance through a 

 process of selection of minor factors. 



INTENSITY 



Turning to intensity, the evidence is 

 clear that white of Yorkshire differs 

 from red of Tamworth by at least one 

 clear-cut doininant factor. There is 

 difficulty, however, in attempting to 

 classify the Berkshires, Poland-Chinas 

 and Hampshires with respect to this 

 series. The fact that the first two 

 ordinarily show only white in ])lace of 

 red, wT)uld lead one to classify them with 

 Yorkshire. The failure of reds to ap- 

 pear when these breeds are crossed twice 

 with Yorkshires is further evidence in 

 this direction. On the other hand. 

 Berkshires and Poland-Chinas may show 

 sandy splotches themselves and in 

 crosses with Tamworths or Durocs pro- 

 duce black and red i)igs in Fi in marked 

 contrast with the whites from York- 

 shire crosses. Similar results were 

 shown in the more complex crosses 

 cited above. 



Some evidence has recently been 

 presented by Severson^^ on crosses 

 between Berkshire and Duroc-Jersey 

 which bears on the genetic relations both 

 as regards intensity of red and exten- 

 sion of black. The offspring of the 

 cross were variable but all intermediate 

 between the parents. Ten were red 

 with varying amounts of black spotting, 

 while two w'cre ])rcdominately black, the 

 parts which were not black Ijeing white 

 in Ihem. Two sows representing ex- 

 treme types in Fi, a red wdth black 

 spots and a blacks with white splotches, 

 were backcrossed w'th a Ik-rkshire boar. 

 The same kinds of young were produced 

 in both cases. There were reds with 

 black spots, blacks with white splotches, 

 and tricolors. These classes graded 



" Sever son, B. (). l'M7. Joik. Hkp., 8:379-381. 



