134 THE SCIENCE OF BIOLOGY 



structures of the embryo appear to be determined by some- 

 thing within this extra-nuclear material of the ovum (Fig. 

 12). But the amazing parallel between the behavior of the 

 microscopic chromosomes and the gross features inherited 

 by the adult is enough to convince the majority of biologists 

 that the chromosome-theory must be accepted as a working 

 hypothesis of the mechanism of heredity. 



The peculiarities of Mendelian inheritance are explicable 

 in terms of chromosomes. Mendel's law may be illustrated 

 by the inheritance of feather color in a cross between two 

 particular varieties of poultry, one of which is black and the 

 other a white with black splashes (Fig. 17 Pi). This is not 

 one of the cases originally described by Mendel, who worked 

 exclusively with plants, but it is one of the best for intro- 

 ductory purposes. When individuals of the black or the 

 white type are bred among themselves the offspring are 

 black and white respectively, showing that black and white 

 are pure. But when one of these blacks is bred with a white, 

 100% of the resulting offspring (Fi) are blue. This seems like 

 blended inheritance. We should naturally expect that blue 

 crossed with blue would give blue in subsequent genera- 

 tions. But Mendelian heredity is not of this nature. 



The result which appears when the Fi blues are bred 

 together illustrates the distinctive feature in Mendelian 

 heredity. If the numbers are sufficient, a theoretical ratio 

 of 1 Black: 2 Blue: 1 White is approximated. The blacks 

 of this F 2 generation give only pure blacks when bred among 

 themselves, the whites only pure whites. A black bird of 

 this generation is as pure black as its black grandparent, 

 and the same is true of the white. Segregation of the black 

 from the white has occurred. The seeming blend of the Fi 

 generation was not permanent, although similar individuals 

 comprise the 50% of Blues in the F 2 . These F 2 blues when 

 bred among themselves give the same results as the Fi blues, 

 namely, 25% Black: 50% Blue: 25% White. The results 

 obtained by breeding these F 3 individuals are as previously 



