Chromosomes and Genes 181 



I realize how diflOlcult it is for the geneticist thinking in terms 

 of individual genes to visualize this. Mather (1946, 1948), who joined 

 me in these deliberations, overcame the mental difficulty by calling 

 the chromosomal sections of genetic action of varying size "fields of 

 cooperation," and used as an illustration the diagram reproduced 

 in figure 13, which is essentially the same as the one above. Of 

 course the word "cooperation," though conveying a rather plastic 

 view of the idea, involves the danger of being understood as additive 

 actions of individual genes. This is not what is meant; rather, it is a 



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 (2) 



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Fig. 13. Mather's discussion of chromosomes and genes: "In 1 and 2 the 

 genes are units of physical structure of the chromosome as well as physiological 

 units of action. They may be likened to either simple ( 1 ) or composite ( 2 ) 

 beads on a string. In 3 and 4 the genes are not delimited in any special physical 

 way. The parts of one gene are related physically in the same way as adjacent 

 parts of neighbouring genes. The gene is then delimited only as a physiological 

 unit, by the special co-operation or integration of its parts in action, as indicated 

 by brackets. In 3 the fields of co-operation, and hence the division of the 

 chromosome into genes, are constant throughout the life cycle. In 4 the fields 

 of co-operation, and hence the division of the chromosome into genes, vary 

 during the life cycle." 



field action which in an electromagnetic field, for example, may also 

 be described in terms of cooperation of the innumerable points of 

 the field. I have occasionally used another simile which may help 

 the visualization if not taken too literally. If we study the effect 

 of light waves upon an organism, we may find definite actions of 

 ultraviolet light of one single wave length which we compare to 

 the workings of a single genetic locus. We might then find actions 

 which are essentially the same over a number of wave lengths, say 

 within the ultraviolet. Other actions may be typical for the entire 

 ultraviolet or other part of the spectrum; still other Hght effects may 



