HEREDITY AND MENDEL's LAW l8l 



First, let us look at a case of color inheritance. If a bird 

 having black feathers, like a Minorca, or one having black 

 feathers mottled with white, like a Houdan, be crossed with a 

 white bird, like the White Leghorn, the male hybrids are pure, 

 or almost pure, white. White dominates more or less com- 

 pletely over black ; but the black is not lost, it is merely over- 

 shadowed, for if two of these white hybrids be mated, about 

 25 per cent, of their offspring will be as black as the original 

 parents. Such black grandchildren derived from two white 

 parents are known as extracted blacks. The black color has 

 been released from the dominance of the white ; and if the 

 extracted blacks be bred together they behave like a pure black 

 race. A characteristic like black which recedes from view in 

 the first hybrid generation, which may be extracted from the 

 dominant type in 25 per cent, of the second generation, and 

 which, after extraction, breeds true, is called a recessive char- 

 acteristic. 



The discovery of the law of inheritance in the case of dom- 

 inant and recessive characteristics was made forty two years 

 ago by an Austrian monk, Gregor Mendel, while experimenting 

 with plants in his cloister garden. Mendel not only discovered 

 his law, but gave a satisfactory cytological explanation of it. 

 He states that the germ cells of the hybrid must divide into two 

 kinds, each pure in respect to two opposed characteristics, the 

 dominant and the recessive. When tw^o of these hybrids are 

 mated there are four equally probable events : an o.^^ contain- 

 ing the dominant character Z^may be fertilized with a D sperm 

 or an R sperm, and an R ^gg may be fertilized by a D sperm 

 or an R sperm. Whenever the character D enters into the 

 combination, that character dominates in the offspring ; but 

 where it does not enter, R shows itself irr the same. The 

 former event occurs in three-fourths of all cases; the latter in 

 one-fourth, consequently only one-fourth of the offspring are 

 recessive. This is in brief outline the essence of the Mendelian 

 doctrine. It rests on the foundation of purity of the germ-cells 

 and hence purity of the one-fourth of the second hybrid gener- 

 ation that is produced by the union DD and the one-fourth 

 resulting from the union RR. These portions, inbred, should 



