•SOMATIC SEGREGATION" 



A Misleading Term, Not Warranted by Knowledge of the Facts The Alleged 



Somatic Segregation in Calyx of Pear — Probably to be 



Explained as a Result of Hybridization 



E. B. Babcock 



Professor of Genetics, University of California, Berkeley, Cal., and 



Francis E. Lloyd 



MacDonald Professor of Botany, McGill University, Montreal, Canada 



THAT the proper use of terms is 

 essential to mutual understand- 

 ing amon^ scientists is too ])atent 

 to require arj^ument. Many of 

 the disputations that have occupied 

 the time of investigators have arisen 

 through the use of the same term in a 

 different sense or of misleading expres- 

 sions in referring to commonly recog- 

 nized phenomena. The literature of 

 genetics, like that of other develo])ing 

 sciences, has not been free from the 

 confusion arising from misuse of terms. 

 In the interest of common progress 

 the authors feel that the time has arrived 

 to call attention to one misleading 

 expression which has come into rather 

 general use, especially among horti- 

 culturists. We refer to the use of the 

 term, segregation, in designating somatic 

 phenomena. This term in Mendelian 

 inheritance means the distribution of 

 allelomor]jhic factors among the gametes 

 formed by a hybrid organism, half of 

 the gametes receiving one and half the 

 other member of each allelomorphic 

 13air. The mechanism by which this 

 segregation is accom])lished is the sepa- 

 ration of whole chromosomes in the 

 heterotyjMC mitosis (meio.sis) during 

 the maturation of the germ cells. 

 Unfortunately the term has also been 

 used in referring to the result of the 

 recombinations of allelomorphs in hybrid 

 progeny and this looseness ])robably 

 has led to the wider de])arture from the 

 strict meaning of the term mentioned 



above. Some writers as, for example, 

 Kraus' in his recent paper on somatic 

 segregation, a]:)i)ear to use the terms 

 segregation and mutation rather loosely. 

 Thus, according to Kraus, "vegetative 

 variations are first of all of two distinct 

 sorts — modifications or fluctuations, 

 which do not remain true when propa- 

 gated and subjected to varying condi- 

 tions, and segregations or mutations 

 which may be pro]jagated and expected 

 to remain reasonably constant under a 

 wide range of conditions." It is perhaps 

 hardly necessary to point out that 

 modifications and fluctuations are no 

 more truly synon^-mous than are segre- 

 gations and mvitations, since some 

 fluctuations are attributable to other 

 causes than environmental effects. 



bateson's suggestion 



Bateson- first suggested the possi- 

 l^ility that segregation might sometimes 

 occur during somatic mitosis. But 

 although cy-tologists have demonstrated 

 several irregularities in chromosome 

 behax'ior during meiosis (when homol- 

 ogous chromosomes normally become 

 dissociated and distributed to different 

 gametes) there is no evidence that 

 chromosome irregularities occur during 

 ordinary cell divisions in embryonic or 

 meristematic tissues, where it would be 

 necessary for segregation to occur in 

 order to jjroduce profound somatic 

 changes. It is conceivable, of course, 

 that the division by longitudinal split- 



'Kraus, K. J. Somatic ScgrcKation. Jorkwi. <ii IIkkkdi rv, \'I I , t, l'M(>. 

 * Bateson, W. Mcmirl's Prin(i])l(s of HiTi-<lity. l')()<^ j))). 272-3. 



82 



