224 



NATURE 



[October 14, 1920 



Our Conceptions of the Processes df Heredity/ 



By Miss E. R. Saunders, F.L.S. 



I. 



OY the term "inheritance" we are accustomed to 

 ■D sijjnify the obvious fact of the resemblance dis- 

 played by all living organisms between offspring and 

 parents, as the direct outcome of the contributions 

 received from the two sides of the pedigree at fer- 

 tilisation ; to indicate, in fact, owing to lack of 

 knowledge of the workings of the hereditary process, 

 merely the visible consequence — the final result of a 

 chain of events. Now, however, that we have made 

 a beginning in our analysis of the stages which cul- 

 minate in the appearance of any character, a certain 

 looseness becomes apparent in our ordinary use of 

 the word "heredity," covering as it does the two 

 concomitant essentials, genetic potentiality and 

 somatic expression — a looseness which may lead 

 us into the paradoxical statement that inheritance is 

 wanting in a case in which, nevertheless, the evidence 

 shows that the genetic constitution of the children 

 is precisely like that of the parents. When we say 

 that a character is inherited no ambiguity is involved, 

 because the appearance of the character entails the 

 inheritance of the genetic potentiality. But when a 

 character is stated not to be inherited it is not thereby 

 indicated whether this result is due to environmental 

 conditions, to genetic constitution, or to both causes 

 combined. That we are now able in some measure 

 to analyse the genetic potentialities of the individual 

 is due to one of those far-reaching discoveries which 

 change our whole outlook, and bring immediately in 

 their train a rapidly increasing array of new facts, 

 falling at once into line with our new conceptions, 

 or by some orderly and constant discrepancy pointing 

 a fresh direction for attack. 



The earliest attempts to frame some general law 

 which would co-ordinate and explain the observed 

 facts of inheritance were those of Galton and Pearson. 

 These schemes, however, take no account of the 

 physiological nature of this as of all other processes in 

 the living organism. They have, in consequence, failed 

 to bring us nearer to our goal — a fuller comprehension 

 of the workings of the hereditary mechanism. Pro- 

 gress in this direction has resulted from the method 

 of inquiry which deals with the unit in place of the 

 mass. The revelation came with the opening of the 

 present century, for in 1900 was announced the re- 

 discovery of Mendel's work, actually given to the 

 world thirty-five years earlier, but at the time leaving 

 no impress upon scientific thought. It chanced that 

 in each pair of characters selected by Mendel for 

 experiment the opposites are related to each other in 

 the following simple manner : .\n' individual which 

 had received both allelomorphs, one from either 

 parent, exhibited one of the two characteristics, hence 

 called the dominant, to the exclusion of the other. 

 Among the offspring of such an individual both 

 characteristics appeared, the dominant in some, its 

 opposite, the recessive, in others, in the proportion 

 approximately of three to one. This is the result 

 which might be expected from random pairing in 

 fertilisation of two opposites, where the manifesta- 

 tion in the zygote of the one comoletelv masks the 

 presence of the other. As workers along Mendelian 

 lines increased and the field of inquiry widened, it 

 soon, however, became apparent that the dominant- 

 recessive relationship is not of universal occurrence. 

 It likewise became clear that the simple ratios which 



* From the openine address of the President of Section K (Rotany) 

 delivered at the Cardiff meeting of the R'itish Association on August 24. 



NO. 2659, VOL. 106] 



obtained in Mendel's experiments are not characteristic 

 of every case. Mendel's own results were all, as it 

 happened, explicable on the supposition that the two 

 alternative forms of each character were dependent on 

 a single element or factor. We now know, however, 

 that many characters are not controlled by one single 

 factor, but by two or more. One of the most familiar 

 instances of the two-factor character is the appear- 

 ance of the colouring matter anthocyanin in the petals 

 of plants such as the stock and sweet pea. Our 

 proof that two factors (at least) are here involved is 

 obtained when we find that two true breeding forms 

 devoid' of colour yield coloured offspring when mated 

 together. In this case the two complementary factors 

 are carried, one by each of the two crossed forms. 

 When both factors meet in the one individual, colour 

 is developed. We have in such cases the solution of 

 the familiar, but previously unexplained, phenomenon 

 of reversion. Confirmatory evidence is afforded when 

 among the offspring of such cross-bred individuals we 

 find the simple 3 to i ratio of the one-factor differ- 

 ence replaced by a ratio of 9 to 7. Similarly, we 

 deduce from a ratio of 27 to 37 that three factors are 

 concerned, from a ratio of 81 to 175 four factors, and 

 so on. The occurrence of these higher ratios proves 

 that the hereditary process follows the same course 

 whatever the number of factors controlling the 

 character in question. 



And here I may pause to dwell for a moment upon 

 a point of which it is well that we should remind 

 ourselves from time to time, since, though tacitly 

 recognised, it finds no explicit expression in our 

 ordinary representation of genetic relations. The 

 method of factorial analysis based on the results of 

 inter-breeding enables us to ascertain the least 

 possible number of genetic factors concerned in con- 

 trolling a particular somatic character, but what the 

 total of such factors actually is we cannot tell, since 

 our only criterion is the number bv which the forms 

 we employ are found to differ. How many may be 

 common to these forms remains unknown. In illus- 

 tration I may take the case of surface character in 

 the genera Lychnis and Matthiola. In Lychnis ves- 

 pertina the type form is hairy ; in the variety glabra, 

 recessive to the type, hairs are entirelv lacking. 

 Here all glabrous individuals have so far proved to 

 be similar in constitution, and when bred with the 

 tvpe give a 3 to i ratio in F, (Report to the Evolu- 

 tion Committee, Royal Society, i., 1902). We speak 

 of hairiness in this case, therefore, as being a one- 

 factor character. In the case of Matthiola incana v. 

 glabra, of which many -strains are in cultivation, it 

 so happened that the commercial material originally 

 emploved in these investigations contained all except 

 one of the factors since identified as present in the 

 tvpe and essential to the manifestation of hairiness. 

 Hence it appeared at first that here also hairiness 

 must be controlled, as in Lychnis, by a single 

 factor. But further experiment reveale<d the fact 

 that though the total number of factors contained in 

 these glabrous forms was the same, the respective 

 factorial combinations were not identical. By inter- 

 breeding these and other strains obtained later, hairv 

 F, cross-breds were produced givinsf ratios in F, 

 which proved that at least four distinct factors are 

 concerned (Proc. Rov. Soc. , B, vol. Ixxxv.. IQ12). 

 Whereas, then, the glabrous appearance in Lychnis 

 always indicates the loss (if for convenience we may 

 so represent the nature of the recessive condition) of 



