462 



GENETICS 



The Method of Experimental Breeding 



Mendel's Contribution. — While the statistical or biometrical 

 method gives a survey of the average course of heredity and varia- 

 tion in populations and pure lines consisting of numerous indi- 

 viduals, it is of no value in the analysis of individual cases. Experi- 

 mental breeding, in a controlled environment, makes possible the 

 accumulation of data on the process of heredity in all the individuals 

 produced from generation to generation. 



The first recorded experiment in 

 breeding was that of Camerarius, 

 in 1694. Not until Father Gregor 

 Mendel (1822-1884) of the August- 

 inian Order, carried out his careful 

 work on the breeding of peas in the 

 monastery garden at Briinn in 

 Austria, did this method yield results 

 that made clearer the principles of 

 heredity. Mendel's success resulted 

 from the fact that he followed the 

 inheritance of single specific char- 

 acteristics in many individuals for 

 many generations. One of his orig- 



FiG. 242.— Gregor Mendel, 1866 i^al experiments was that of cross 

 „ , , , ... fertilization between peas with tall 



Enlarged from a group of the _ '^ 



brethren of the Konigskloster. (From stcmS and thoSC with dwarf stcmS. 



redi";/'otp;tH. .mty'c.^! The use of parents that differ with 



bridge University Press, reprinted by respect tO OUe Or mOre characteristics 



P""^'^^°'°°^ is known as ''hybridization," and 



the offspring of such a cross fertilization are " hybrids." If the 

 parents differ with respect to one characteristic, such as length of 

 stem in peas, a cross between them is called mono-hybridization. 

 Mendel found that the offspring produced by hybridizing tall and 

 dwarf peas were, without exception, tall (Fig. 243) whether the 

 male parent was tall and the female dwarf, or vice versa. When 

 these tall hybrid peas were crossed among themselves, three-fourths 

 of the next generation were tall, while one-fourth were dwarf hke 

 their dwarf grandparents. Further breeding showed that the 

 dwarf progeny of the tall hybrids gave only dwarf offspring when 

 interbred. That is, the characteristic of the dwarf stem, which 



