CEREAL IMPROVEMENT HYBRIDIZATION 211 



nance. A hybrid of black and white oats breaks up in 

 the second generation into 25 per cent black, 25 per cent 

 white, and 50 per cent brown, because of the dominance 

 of black color, but in the next generation half of the 

 brown hybrids are split again into absolute black and 

 white. These unit-characters are often called segregat- 

 ing or Mendelian characters. The fixity of unit-char- 

 acters has been disputed by some. 1 



221. Dominants and recessives. : The application of 

 Mendel's Law is best seen when the two parents possess 

 characters that are opposed to each other, as in the in- 

 stance of awned and awnless wheats. In such a case the 

 opposing characters are termed a character pair. If one 

 of the characters is stronger, it is said to be dominant, 

 and it alone will appear in the first generation, the other 

 remaining concealed or recessive. Presence of awns is 

 recessive, and therefore completely awned plants will not 

 appear until the second generation, but the progeny of 

 these will afterward breed true. There is evidently some 

 advantage in knowing what characters are recessive, as 

 hybrids showing such a character in the second generation 

 are at once fixed as to that character. Some examples of 

 dominant characters and their corresponding recessives 

 are as follows : 



Dominant characters : Corresponding recessives : 



Spikes awnless Spikes awned 



Red chaff White chaff 



Gray chaff White chaff 



Black hulls (oat kernels) White hulls (oat kernels) 



Red kernels White kernels 



1 See Castle, W. E., The Inconsistency of Unit Characters, 

 Am. Nat. XLVI, 352-363, 1912. Also Pure Lines and Selection, 

 Jour. Hered. V, 93-97, 1914. 



