132 THE CORNELL READING-COURSES 



two thirds produced tails and dwarfs in the original ratio of three of 

 the former to one of the latter. All the dwarf plants produced only 

 dwarfs. 



In this particular experiment two unit characters were considered, 

 namely, tallness and dwarfness. The first- is the dominant character; the 

 second is the recessive. When these characters are possessed by two 

 different plants that are crossed, the first generation after the cross will 

 contain both of them. Owing to its dominance, only the character of 

 tallness seems to be present. The recessive character, dwarfness, is sub- 

 merged or hidden, but appears in the second generation after crossing, 

 in one fourth of the individuals. This appearance of the recessive along 

 with the dominant is what is meant by segregation. The reader must 

 not think that segregation occurs always in the exact ratio of three to 

 one. Perhaps it never comes out numerically exact. But if a large number 

 of individuals are grown, on the average the ratio will approximate three 

 of the dominants to one of the recessives. 



For the sake of simplicity, the writer has used only the two characters 

 of peas to explain dominance, recessiveness, and segregation. It has been 

 worked out with innumerable plant characters since Mendel's time, as 

 well as with many animal characters. In the following list, which contains 

 only a few of the many examples that might be drawn upon, the dominance 

 for each pair of characters has been definitely determined. In each case 

 the dominant character is given first. 



1. Tallness and dwarfness. Peas 



2. Rounded and wrinkled seed. Peas 



3. Yellow and green seed. Peas 



4. Smooth and bearded heads. Wheat 



5. Long and short staple. Cotton 



6. Susceptibility and resistance to rust. Wheat 



7. Starchy kernels and sugary kernels. Corn 



8. Yellow kernels and white kernels. Corn 



The reader's attention is directed especially to number 6 in the list. 

 Here it is evident that susceptibility and resistance to disease behave as 

 unit characters. This fact was discovered by Professor Biffen of England, 

 who has done considerable work with wheat. 



The practical man may wonder of what service Mendel's law may be, 

 if various characters of plants and animals brought together in a hybrid 

 merely separate according to a definite ratio in the second and following 

 generations. A few words of explanation may be helpful. In addition 

 to ordinary segregation, there may occur a recombination of characters. 



