232 PRINCIPLES OF POULTRY BREEDING 



D, being the dominant character, will be present in three- 

 fourths of the progeny. In one-fourth of the progeny represented 

 by D 2 , the character will be pure. In one-half represented by 

 2 Dr, it will be mixed with the recessive, but, owing to its 

 dominance, will suppress the recessive (as to appearance). The 

 other one-fourth of the progeny, R 2 , will have only the recessive 

 character. 



If the breeding of individuals resulting from the first cross be 

 continued, interesting phenomena of this law will appear. D 2 

 and R 2 will continue indefinitely to breed true if bred to them- 

 selves, as they respectively contain only pure dominant and pure 

 recessive characters. Mendel's law refers specially to the hybrids 

 or the apparently crossed portion of the progeny represented 

 by 2 Dr. 



When individuals of the Dr group are bred together, their 

 offspring will in turn be split in the same algebraic ratio of 



D 2 +2 Dr+R' 



which was apparent in the first cross when pure forms were allowed 

 to breed together. This ratio will continue indefinitely when the 

 hybrids of future generations are bred together. The same will 

 hold true of the dominant and recessive characters possessed by 

 the hybrids, namely, that the dominant character will always 

 show in the Dr group even when the recessive is also present. 

 In this second generation D 2 and R 2 are pure and will breed pure. 

 (They are technically called homozygotes.) The other individuals 

 resulting from the same cross, represented by the letters 2 Dr, 

 are hybrids like their parents (and are technically called hetero- 

 zygotes). 



From the above discussion it will be seen that, since the Dr 

 group, or mixed group, always exhibit the dominant character, it 

 is often a difficult problem to differentiate them from the dominant 

 group D 2 



To test a dominant, mate it to a recessive, and if all the offspring 

 are pure dominants no recessive character will appear. If this 

 parent is not pure, but mixed, the offspring will be one-half domi- 

 nant and one-half recessive, having followed the combination: 



(Dr) + (RR). 



All hybrids which produce only pure dominant characters may 



