HEREDITY AND VARIATION 



The Method of Experimental Breeding 



Mendel's Contribution. Although the statistical or biometrical method 

 gives a survey of the average course of heredity and variation in populations 

 and pure lines consisting of numerous individuals, it is not of value in the 

 analvsis of individual cases. Experimental breeding in a controlled environ- 

 ment makes possible the accumulation of data on the process of heredity in 

 all the indi\'iduals produced from generation to generation. The first re- 

 corded experiment in plant breeding was that of Camerarius in 1694. Not 

 until Father Gregor Mendel (1822 1884) of the Augustinian Order carried 

 out his careful work on the breeding of peas in the monastery garden at Brno 

 (Briinn), Moravia, did this method yield results that revealed the principles 

 of heredity (Fig. 6.1). Mendel's success came when he followed the inheri- 

 tance of single specific characteristics in many individuals for several genera- 

 tions. One of his original experiments was cross-fertilization between peas 

 with tall stems and those with dwarf stems. The use of parents that differ 

 in one or more characteristics is known as the method of hybridization, and 

 the offspring of such a cross-fertilization are hybrids. If the parents differ 

 in one characteristic, such as length of stem in peas, a cross between them is 

 called monohybridization. Mendel found that without exception the offspring 

 produced by hybridizing purebred tall and dwarf peas were tall, no matter 

 which parent was tall and which dwarf (Fig. 6.2). When these hybrid tall 

 peas were crossed among themselves, three-fourths of the next generation were 

 tall like the tall parents, and one-fourth were dwarf like their dwarf grand- 

 parents. Dwarf stem, a characteristic which did not appear in the first filial 

 generation (F^ generation) that arose from the cross between tall and dwarf 

 parents (P^ generation), emerged unchanged in the second filial generation 

 (^2 generation). Dwarfness in such peas was as pure as was the dwarfness of 

 their grandparents, and these F, dwarf peas gave rise only to dwarf peas when 

 they were bred together. Breeding of the tall peas that constituted three- 

 fourths of the F2 generation revealed that, although these tall individuals 

 superficially resembled one another, they were dissimilar as parents. One- 

 third of the tall group gave rise in successive generations to tall offspring 

 without exception. Such tall specimens, which constituted one-fourth of the 

 total F2 generation, corresponded, in their resemblance to the purebred tall 

 pea of the P^ generation, to that quarter of the group that was like the 

 dwarf pea of the P^. The remaining two-thirds of the tall peas of the F2 gen- 

 eration, or one-half of the offspring of the hybrid tall peas, were like their 

 parents. When interbred, these F2 tall peas gave rise to offspring in the 

 ratio of three tall to one dwarf. As shown in Figure 6.2, this group again 

 breaks up, when analyzed by breeding, into three types that occur in the ratio 

 of 1 : 2 : 1 ; that is, one-fourth are pure tall peas, one-half hvbrid tall peas, and 

 one-fourth pure dwarf peas. In the case under discussion, tallness is said to 

 be dominant to dwarfness; conversely, dwarfness is recessive to tallness. 



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