December 15, 1911] 



SCIENCE 



841 



DISCUSSION AND COSBESPONDENCE 



•J " GENOTYPE " AND " PURE LINE " 



The widespread interest in the lectures on 

 genetic problems now being given in this 

 country by Professor "W. Johannsen makes it 

 worth while to point out certain diversities in 

 the usage of terms introduced by him — di- 

 versities giving an appearance of disagree- 

 ment where none exists. The fact that the 

 present writer is partly responsible for any 

 confusion thus caused impels the publication 

 of this note. 



The term genotype was introduced by Jo- 

 hannsen in connection with the term pheno- 

 type. The latter designates a group of organ- 

 isms which in outward appearance seem to be- 

 long to one type, although in hereditary consti- 

 tution they may actually differ greatly. 

 Genotype, in Johannsen's usage, is not di- 

 rectly contrasted with phenotype, to signify a 

 group of organisms that actually do possess in 

 all respects the same hereditary constitution — 

 though this is the sense in which some of us have 

 been using it. It arose as follows. Organisms 

 with hereditarily different constitutions must 

 have different combinations of the deter- 

 miners, called by Johannsen genes, that decide 

 what the somatic characters shall be. They 

 have, then, different typical combinations of 

 genes. Johannsen calls the particular com- 

 bination of genes that an organism has, its 

 genotype. Or, without reference to genes, we 

 might say that the genotype of any organism 

 is the particular combination of hereditary 

 features that characterize it. Thus, as em- 

 ployed in Johannsen's usage, genotype is an 

 abstract term. 



When a group of organisms all have demon- 

 strably the same combination of hereditary 

 characteristics, one can say that they have the 

 same genotype, or that they belong to the 

 same genotype. From this it is but a step to 

 the employment of the word as a name for 

 such a concrete group of organisms, all with 

 the same hereditary characteristics. Follow- 

 ing a bent toward concreteness, I have used 

 the term in this way in my paper on " Pure 

 Lines in the Study of Genetics in Lower Or- 



ganisms." ' ShuU has done the same in his 

 paper on the " Genotypes of Maize " ;° appar- 

 ently this use of the term for a concrete, vis- 

 ible group of organisms is becoming general; 

 for a term with this precise meaning is much 

 needed. But this is not the usage of Johann- 

 sen. 



Thus arise such differences as that shown by 

 my own characterization of genotypes as 

 " concrete realities," as " actual existences 

 that strike you in the face," etc.,' when com- 

 pared to Johannsen's statement that " we do 

 not know a genotype," etc.,' and that this is a 

 concept with which we cannot actually oper- 

 ate.'' There thus arises an appearance of 

 opposition where none exists. What I and 

 some others have called a genotype is what 

 Johannsen would call a group of organisms 

 " identical in genotypical constitution." The 

 usage recommended by the originator has of 

 course the right of way." 



A diversity of usage likewise exists as to the 

 expression " pure line." I have employed this 

 to designate a genealogical series in which 

 there arises no diversity in hereditary char- 

 acteristics, either from within or from with- 

 out; such, for example, as the series produced 

 by the repeated fission of a single infusorian. 

 Pure lines in this sense might be expected, 

 from what we thus far have learned, (1) in 

 eases of vegetative reproduction, (2) in at 

 least some cases of parthenogenesis (where no 

 reduction division occurs), (3) in case of self- 

 fertilization of homozygotic organisms, (4) in 

 case of inbreeding of a group of genotypically 

 identical homozygotic organisms. 



The pure lines investigated by Johannsen 

 fall in the third group, and he employs their 



^ Amer. Nat., February, 1911. 



' Amer. Nat., April, 1911. 



= X. c, p. 80. 



*Amer. Nat., March, 1911, p. 134. 



^ ' ' Elemente der exakten Erblichkeitslehre, ' ' p. 

 130. 



" Whether the word itself should be given up, in 

 this connection, because it had earlier been used 

 in an entirely different sense, is of course a dif- 

 ferent question. 



