556 



BREEDING 



BREEDING 



iMder*<J in connection with these same 100 unions, there would occur 

 the foUowinK couibinutions, acconiinR to the law of chance: 



2o SS 



12>3SlU>e 

 6h SeSe 



50 Sy 



12 ij SUyR 

 25 SHyc 

 12 >3 Seye 



I'o j-y 



6 U yRyR 

 12 ' 2 yRyo 



•> 'A yeye 



These nine combinations are the same as given above, but the 

 percenlase of each combination out of the 100 unions is shown. 



If a third character were considered, the proportions of the 

 combinations can be determined in exactly the same way. Each 

 one of the above nine p*>ssible combinations would be again divided 

 into three different unions in the same way as the three combina- 

 tions of the one character-pair gave nine different combinations 

 with the second character-pair. In the consideration of the three 

 character-pairs, there would thus be twenty-seven different com- 

 binations of parental characters. .\nd again in each ovary fecun- 

 dated, when only one determiner of each character-pair occurred, 

 the opposing character-determiner being in each case eliminated, 

 such a cell should give a plant that would reprod\ice its character 

 true to t>'pe. It is well known that almost any two different races 

 or species that may be chosen for hybridization will ordinarily 

 differ from each other in numerous characters. When there are a 



647. Pepper hybrids distinct from parental types, formed by 

 recombination of characters; a, dwarf type with few small hori- 

 zontal branches; b, giant type with many large erect branches. 



number of these opposing characters which form Mendelian 

 character-pairs, the aetermination of the possible combinations by 

 Mendel's formulae becomes very complex and difficult to under- 

 stand. It is only by taking a few well-marked character-pairs and 

 carefully studying them that the segregation and new combi- 

 nations according to Mendelian proportions can be followed and 

 understood. 



Any character-pairs following Mendel's law would segregate aa 

 indicated aixive. in the case of scarlet and yellow fruits and reversed 

 or erect fruits of the pepper. A very large number of characters of 

 various plants and animals are now known to be Mendelian and 

 while many modifications of the principles have been necessary to 

 harmonize them with special ca-ses, still it may be said that there is 

 no other general law of heredity and Mendel's law has thus fur- 

 ni.nhed us with a working basis of great value. 



Tfje study of hybrids has been resolved into a study of unit- 

 characters and their relation to each other. Uy hybridizing related 

 types having opposed characters and observing the segregations 

 which o<:cur in the later generations, the characters of each type 

 are analysed and it is determined when a character-p.uir occurs. 

 The rev-arches on this subject by Mendel. Uateson, Davenport, 

 Caitle. Punnett, .Shull. Hurst, Correns, "Tschermak, Kast and dozens 

 of other now well-known investigators, have developed a science 

 of heredity of which there was no conception a few years ago. 



The characters prcs<;nted by the different varieties of a plant or 

 of different .ijM:cie», which can be crossed with it, can now be studied, 

 and one can dehnitely plan tlie combination of characters desired 

 m an ideal type, and can with considerable confidence estimate the 

 number of plants it will be necessary to grow to get this combina- 

 urm. It is now known in general how characters behave in segre- 

 gation and inheritance, so that one can go about the fixation of a 

 deoired type, wbeo one is secured, in an orderly and intelligent 

 way. 



The further the study of cliaructers is carried, the more it is 

 coming to be realized that the appearand' of apparently new types 

 following hybridization is due to reeoiiil>in:iti">ns of different units 

 which in tlicir reactions give apparently lu'w characters. As an 

 illustration, in a study of pepper hybrids, which has been con- 

 ducted during the past four years, it has become evident that the 

 form of plant and branching is due to three pairs of characters or 

 allelomorphs; namely, first, erect or horizontal branches; second, 

 large or small hniiu-tics; and third, many or few branches. In cross- 

 ing two medium-sized races, one with large horizontal and few 

 branches, and the other with small erect and numerous branches, 

 there result many new combinations of characters, among which 

 appear some with small horizontal and few branches, which gives 

 a dwarf plant, and others will have a combination of large erect 

 and numerous branches, which gives a giant plant (Fig. 647). 

 These dwarfs on the one hand and giants on the other appear as 

 distinct, new creations, though they are very evidently merely the 

 recombinations of already existing unit characters, and dwarfness 

 and giantness are the results of the reaction of the different units 

 combined. 



When the large number of distinct characters that are pre- 

 sented by the very numerous varieties of any of our cultivated 

 plants is remembered, an understanding is secured of the pos- 

 sibilities of improvement which the field of hybridization affords. 



The development of hybrids into pure races. 



When hybrids have been produced between species or 

 varieties possessing certain characters that it is desired 

 to unite in a variety, the recombinations of characters 

 as explained in the preceding section become visible 

 in the second generation, and it is thus among the plants 

 of this generation of the hybrid that one should expect 

 to find the combination of characters desired. The 

 breeder would thus very carefully examine a large num- 

 ber of second-generation plants and choose for further 

 experimentation those plants that were found to have 

 inherited the characters which he desired to combine. 

 The entire batch of F2 plants should be carefully exam- 

 ined to determine what characters behave as character- 

 pairs and also the dominant or recessive nature of each 

 character. This knowledge is necessary in order to 

 determine the practice to be pursued in choosing plants 

 in which the characters desired will be pure with refer- 

 ence to these characters. If, for example, the breeder is 

 working to get a combination of two characters only, 

 such for instance as a yellow- and erect-fruited pepper, 

 from the combination of character-pairs discussed abo^'e 

 in explaining Mendel's law he would discover that both 

 of these characters are recessive, and thus when a hybrid 

 was found in which these two characters were united, 

 he couki be sure that by self-fertilizing such an individ- 

 ual it would reproduce true with reference to both of 

 these characters in the next and succeeding generations. 

 He would know furthermore in dealing with only two 

 pairs of characters that he should, according to the law 

 of chance, secure on an average about one such com- 

 bination in sixteen hybrids. 



If, however, the combination desired was a scarlet 

 reversed fruit, both dominant characters, the process 

 would be much more difficult. As shown in the preced- 

 ing section describing the segregation and recombina- 

 tion of characters, nine plants out of the sixteen possible 

 combinations would have red, reversed fruits, while 

 only one of the nine would be pure with reference to 

 both of these characters. The breeder would thus be 

 compelled to self-fertilize a niunber of the plants hav- 

 ing red and reversed fruits and grow a number of plants 

 from each in order to determine which one, if any, was 

 pure with reference to both characters. If, then, the 

 progeny from any one of the plants chosen and self- 

 fertilized came true to t^pe with reference to both 

 characters, he would be certain of its purity and would 

 again self-fertilize some of the best plants of this 

 progeny, whiiih should give him a pure type. 



If a combination of a dominant and recessive charac- 

 ter is desired, the examination of the F2 hybrids would 

 enable the breeder to choose a pure plant so far as the 

 recessive character is concerned, but he could not 

 determine the purity of the dominant character and 

 would be compelled to self a number of plants exhibit- 

 ing the two characters and grow progenies in the third 



