Babcock and Lloyd: "Somatic Segregation" 



may not be of much significance in 

 this case, since continuous variation is a 

 normal feature of the expression of some 

 single genetic factors. Moreover, Jen- 

 nings,^ in his recent contribution on 

 heredity, variation and the resvdts of 

 selection in Difflugia corona, reaches the 

 general conclusion that a single stock, 

 derived by fission from a single pro- 

 genitor, gradually differentiates into 

 hereditarily diverse stocks, so that by 

 selection marked results are produced. 

 In discussing his resiilts, he points out 

 that while large steps do occur, "much 

 more frequent are very slight inherited 

 changes, not fully inherited, and giving 

 a slow alteration of the stock with the 

 passage of generations." Also that, 

 while an inherited variation does involve 

 a chemical change, "the question of 

 continuity or discontinuity in the nature 

 of hereditary variation is not one of 

 observed fact." 



NECESSARY ASSUMPTIONS 



With reference to the point at issue, 

 as to whether or not this may be con- 

 sidered a case of "somatic segregation," 

 we find that certain fundamental as- 

 sumptions are implied by an affirmative 

 decision in the matter. Because some- 

 thing like a 3 to 1 ratio happens to be 

 found in the relative frequencies of 

 deciduous and persistent calyx lobes 

 on a certain tree, is this a good reason 

 for concluding that "somatic segrega- 

 tion" has taken place in that plant? 

 To reach such a conclusion from the 

 given premise, it is necessary to assimie 

 three things: (1) The tree must be a 

 hybrid between two forms, one of 

 which was homozygous for a factor 

 for deciduous lobes, the other for per- 

 sistent lobes. (2) One of each of these 

 factors must be present normally in each 

 cell of the hybrid plant. (3) Since we 

 know that exfoliation of a calyx lobe is 

 conditioned by failtue or retardation of 

 the secondary growth of the cells of the 

 lobe, we must assimie that in about 

 one-fourth of the lobes on this tree the 

 cells lack the dominant factor, and that 



this abnormal distribution of factors 

 (and hence chromosomes) takes place 

 previous to the mitotic divisions which 

 occur during the development of the 

 lobes. 



In regard to the first assumption, 

 Tufts points out that it is commonly 

 thought that the Le Conte pear is a 

 hybrid between Pyrus sinensis and 

 P. communis and we may grant the 

 assumption. However, it is interesting 

 to note that P. sinensis is described as 

 not entirely constant in its character 

 of having deciduous calyx lobes, while 

 according to Tufts' Hst of varieties about 

 30% of the seedlings of P. communis are 

 also inconstant as. regards the character 

 of persistent calyx lobes. But inasmuch 

 as some unit characters are variable in 

 expression we may let the first assump- 

 tion stand for the sake of argument. 

 The second assumption follows of 

 necessity, if we admit that the Le Conte 

 pear is a sexually produced hybrid. 

 It is in regard to the third assimiption 

 that we must take issue. The general 

 regularity of the process of mitosis is 

 sufficient argument against it. While 

 an occasional aberration in chromosome 

 behavior might occur in somatic cell 

 divisions, the occurrence of thousands 

 of such irregularities in the same 

 individual, year after year, is hardly 

 conceivable. 



Therefore we must reject "somatic 

 segregation" as an expression for the 

 designation of phenomena of this sort. 

 Whether a brief general expression can 

 be formulated which will triily represent 

 the facts in this particular case remains 

 to be seen. We may, at least, attempt 

 to state an explanation which is in 

 harmony with the facts of variation. 



In the first place, it must be recog- 

 nized that variability is not necessarily 

 of genetic significance, particularly in 

 such variations as occur in the expression 

 of a character in a single individual. 

 Character expressions may be variable 

 in pure races and also in Fi hybrids 

 which are heterozygous for a single 

 allelomorphic pair. Examples of varia- 



* Jennings, H. S. Heredity, Variation and the Results of Selection in the Uniparental Repro- 

 duction of Difflugia corona. Genetics, Vol. 1, No. 5, 1916. 



