[Reprinted from SCIENCE, N. S., Vol. XLIL, No. 1079, Pages 800-80S, September 3, 1915] 



ARE EECESSIVE CHARACTERS DUE TO LOSS? 



SINCE the presence-absence theory came into 

 vogue it has become quite customary to re- 

 gard recessive characters as due to the absence 

 of something in the germ plasm on which the 

 corresponding dominant character depends. 

 The nomenclature of the presence-absence 

 theory has been adopted by most writers on 

 Mendelian inheritance, and it has afforded a 

 useful and convenient method of expressing 

 gametic formulas, although, as Morgan has 

 shown, there are cases in which it leads to in- 

 consistent results. While it is often recog- 

 nized that this nomenclature is a purely sym- 

 bolic scheme of indicating how certain char- 

 acters behave in inheritance, the habitual em- 

 ployment of the system in the search for form- 

 ulas which will designate by a series of large 

 and small letters the gametic constitution of 

 the organisms one is dealing with, has a strong 

 tendency to influence one's views in regard to 

 several important problems of heredity and 

 evolution. I can not but think that the opin- 

 ions of many students of genetics have been 

 unduly influenced by their formulas. Form- 

 ulas are excellent servants but bad masters. 

 Almost involuntarily a certain interpretation 

 is attached to their symbolism which is apt to 

 have the practical effect of actual belief if it 

 does not succeed in producing it. 



Since the establishment of Mendel's law and 

 its successful employment in elucidating many 

 previously enigmatical phenomena of inherit- 

 ance, heritable variations have commonly 

 come to be considered as due to the addition or 

 subtraction of discrete units of germ plasm, 

 the bearers of unit characters. Professor 

 Bateson in his "Problems of Genetics" says 

 in regard to substantive variations that 



we are beginning to know iri what such variations 

 consist. These changes must occur either by the 

 addition or loss of factors. 



And further on he makes the following sig- 

 nificant statement : 



Eecognition of the distinction between dominant 

 and recessive characters has, it must be conceded, 

 created a very serious obstacle in the way of any 

 rational and concrete theory of evolution. While 

 variations of all kinds could be regarded as mani- 

 festations of some mysterious instability of organ- 

 isms this difficulty did not occur to the minds of 

 evolutionists. To most of those who have taken 

 part in genetic analysis it has become a permanent 

 and continual obsession. With regard to the origin 

 of recessive variations, there is, as we have seen, 

 no special difficulty. They are negative and are 

 due to absences, but as soon as it is understood that 

 dominants are caused by an addition we are com- 

 pletely at a loss to account for their origin, for we 

 can not surmise any source from which they have 

 been derived. 



In his more recent address before the Brit- 

 ish Association, Bateson not only interprets 

 all recessive characters as due to loss, but sug- 

 gests that dominant characters may have 

 arisen by the removal of inhibiting factors, 

 thereby causing a " release " of the characters 

 which previously lay latent in the germ plasm, 

 and producing the appearance (but only the 

 appearance) of new variations. He says: 



In spite of seeming perversity we have to admit 

 that there is no evolutionary change which in the 

 present state of our knowledge we can positively 

 declare to be not due to loss. 



If we explain not only the actual disappear- 

 ance of characters as caused by germinal loss, 

 but the appearance of new characters as due to 

 the loss of inhibitors which prevented these 

 characters from manifesting themselves, it is 

 theoretically possible to consider the whole 

 process of progressive evolution as accom- 

 plished by the sloughing off of inhibiting fac- 

 tors. Such a doctrine which naturally re- 

 minds one of the extravagancies of the theory 



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