356 APPENDIX 



cause the black is stronger than the white and obscures it. 

 The obscuring character is known as a dominant, while the 

 one which appears to be lost is called a recessive. When 

 these crossed young are bred together, however, it is found 

 that, on the average, of every four of the offspring, three 

 are black and one white. Further experiment shows that 

 one of the blacks is pure, and when mated with a specimen 

 of equal purity, will breed nothing but black. The white 

 recessive is equally single in character. The two remaining 

 blacks, however, while often indistinguishable in appearance, 

 are found, when bred together, to again produce blacks and 

 whites in the ratio of three to one. This is due to the 

 fact that they contain equal quantities of black and white, 

 the white being overcast by the dominant black. Such indi- 

 viduals are known as heterozygotes, and so long as they are 

 bred together will continue to produce young in the same 

 proportion of three to one. 



An excellent example is found in blue fowls. When two 

 birds of this color are bred together, young are produced 

 in the proportion of one black, two blues and one white 

 (usually splashed with black). If the black and the white 

 are crossed, all of the young are blue; if the two blues are 

 bred together, blacks, blues and whites again occur in the 

 same proportion.* It is thus shown that blue in fowls is 

 a heterozygous color and never can be permanently estab- 

 lished. This is one of the few cases in which the hetero- 

 zygotes may be distinguished at sight from the pure domi- 

 nant. 



There is one important point in connection with this 

 form of transmission. In most cases characters are inherited 

 singly, and not in relation to others. Thus while an indi- 

 vidual may be dominant so far as one or more characters 

 go, it may be recessive in others. A cross between a black 

 Tunnett, R. G. : Mendelism (American Edition), p. 30. 



