SOME lU.MAHKS ABOUT UNITS IN IIHMEÜITY l.'i? 



sent state of researcli, we have especially to do with such genotypical 

 units as are separable, be it independently or in a more or less 

 mutual linkage. Certainly by far the most comprehensive and most 

 decisive part of the whole genotype does not seem to be able to segregate 

 in units; and as yet we are mostly operating with .characters , which 

 are rather superficial in comparison with the fundamental Specific 

 or Generic nature of the organism. This holds good even in those 

 frequent cases where the characters in question may have the greatest 

 importance for the welfare or economic value of the individuals. 



We are very far from the ideal of enthusiastic Mendelians, viz. the 

 possibility of dissolving genotypes into relatively small units, be they 

 called genes, allelomorphs, factors or something else. Personally I be- 

 lieve in a great central something» as yet not divisible into separate 

 factors. The pomace-flies in Morgan's splendid experiments continue 

 to be pomace- flies even if they lose all »good» genes necessary for a 

 normal fly-life, or if they be possessed with all the »bad» genes, detri- 

 mental to the welfare of this little friend of the geneticists. 



Disregarding this (perhaps only provisional?) central »something» 

 we should consider the numerous genes, which have been segregated, 

 combined or linked in our modern genetic work. What have we 

 really seen? The answ^er is easily given: We have only seen Differences. 

 The famous relation 3:1 (1:2:1) indicates one single point of diffe- 

 rence, the ratio 9:3:3:1 two points, and so on. Dominance does not 

 at all indicate the presence of some positive unit, just as little as Reces- 

 sivity indicates the lack of any unit. This is clearly seen, for instance 

 in Nilssox-Ehle's oats-crossings, where one Mendelian unit may be 

 responsible for one dominant and one or two recessive characters, also 

 in such cases where dominance or recessivity is dependent upon exter- 

 nal conditions, as in some Dro5op/j z7a-experimcnts. 



In the beginning of our modern »Mendelian era» one unit might 

 be regarded as the unit of one descriptive character, for instance 

 »yellow» in ripe peas, »starchy» in maize grains and so on. But when 

 more complicated segregations were found, we conceived the idea of 

 »construction»; for instance of colour and hoariness of stocks. In this 

 case (quoted here as simplified as possible) each of the genes A, B and 

 C when alone (i. e. without the others as elements in the genotype) 

 shows no obvious reaction; but A -\- B may cause the production of 

 colour and .4 + ß + C colour and hoariness, A + ß as well as ß + C 

 giving no observed reaction. 



In such cases we spoke — and may perhaps continue to speak — 



