Nuvemtx-j . 191M 



S( •||-;.\TI|-IC AdJUCl-l.TlllK 



it was quite iipi)aiciit tluit irrespective of 

 the size of beau ehoseii for propagation 

 "vvithin a giveu line, the progeny maintain- 

 ed the weight, eharaeteristic of the line to 

 Avhich they belonged. Sucli variation as 

 did (leeiir was merely fortuitous and se- 

 ie<-tion within a line was consequently 

 without avail in changing the type of the 

 offsjn-ing. Continuing the experiment b}' 

 selecting in successive years large beans on 

 the one hand and small beans on the othei- 

 within a single line. Johainiseu confirmed 

 his contention that these variations were 

 not inherited and that eacli strain con- 

 stituted a pure line which he defined as 

 "the i)rogeny of a single self- fertilized in- 

 diAidual (if hnmogeneous factorial compo- 

 siti(m.'' 



The composition of plant populations in 

 such self-fertilized or autogamous forms 

 as wheat, oats, barle} ', beans and peas now 

 became readily ajij^areut. Since Mendel 

 liad sh(»wii in his origiiud paper that con- 

 tinued self-fertilization resulted in the ra- 

 pid elimination of heterozygosity, it fol- 

 lowed that such species as behaved in this 

 way comprised a number of pure lines and 

 en the isolation of any single plant the odds 

 were overwhelmingly in favor of obtaining 

 a true breeding individual among whose 

 offspring selection would be without effect. 



Various autlioi s in later years liave elab- 

 orated upon the principle involved. Jen- 

 nings '^ has sliown that in the simplest case 

 i. e. in the cross A A x aa giving an Fi 

 generation of the constitution Art, after n 

 genei-ations (if self-fertilization tlie propor- 

 tion of the different genotypes AA Art : 



2^-1 



2°-l 



a a will be 



2° 



respectively. 



Where more than one character pair is in- 

 volved East and Jones- give a convenient 

 formula for calculating the genotypic ex- 

 pectation. If )i represents the number of 

 allelomorphic pairs involved the probable 

 number of homozygotes and any particular 

 ■ class of heterozvgotes in the rth generation 



la Jenning.s. H. S. Xiimerical Results of Diverse 

 Systems of Breeding. Genetics, 1916. 1 pp. 71. 



- East, E. M. and .Jones. D. F. Inbreeding- and 

 Out breed int.'. Phila.. 1919. pp. 90-91. 



is foujid by exi)andiMg tiie binomial 

 1 + (2' - 1)" the exponents of the first 

 term giving the number of heterozygous, 

 and the exponents of the second term the 

 number of homozygous pairs. 



These genetic principles have important 

 applications in crop breeding and in the 

 la.st twenty years have been a most im- 

 portant factor in the development of plant 

 breeding methods. The Swedish In.stitute 

 for the Improvement of Field Ci'ops at 

 Svahif was the first organization of its 

 kintl to practise the isolation of single 

 plants in the production of better varieties 

 of wheat, oats, barley, vetches and peas. 

 Xilsson in 1891 was led to adopt the sep- 

 arate cultui-e system through the accidental 

 discovery that only the offspring of single 

 wheat plants were entirely uniform. The 

 researches of Johannsen having demon- 

 strated the biological soundness of geno- 

 typic selection and defined its limitations, 

 the isolation of pure lines where improve- 

 ment in self-fertilized crops was desired 

 came into extensive use at agricultural ex- 

 periment stations t;iiroughout the world. 

 The genotype hypothesis, fundamental as 

 it is in all plant breeding schemes, forms 

 a notable contribution of science to clear 

 thinking and intelligent practice. 

 II 



In the application of the genotype hy- 

 pothesis to crop improvement schemes, the 

 legitimate question arises if changes may 

 not occasionally take place within pure 

 lines that will so lower their commercial 

 value as to necessitate further selection or 

 a change of seed in t)rder to maintain the 

 original qualities. At least one well known 

 seed growers ' association " has made provi- 

 sion for lemedial measures in case of de- 

 terioration or degeneration in some self- 

 fertilized crops. The remainder of this 

 paper will be devoted to a consideration of 

 the biological justification for such pre- 

 cautions with such species as oats, barley, 

 wheat, beans and peas. 



As applied to the problem in hand we 

 may con.sider the terms deterioration or 

 degeneration as designating any heriiahle 

 cluinse occurring in the stock tending to 



" See Report Canadian Seed Growers' Asso- 

 ciation 15th and 16th years work 1918-1920. 

 paare 6. 



