Genie Control of Development 397 



arrange the major eflFects in a quantitatively increasing series. But if 

 other pleiotropic effects are considered, these may follow the same 

 seriation or they may have a very different order. These facts were 

 much discussed a generation ago, when the problem was whether or 

 not multiple alleles are dosage differences of genes. (See, e.g., pro, 

 Goldschmidt, 1927; contra, Dobzhansky, 1930a; and other material in 

 Stern, 1930.) They are no longer relevant for this problem, since 

 multiple alleles are known to be of very different types, with dosages 

 entering only in the form of potencies of action, as we have seen. 

 However, even if a series of alleles produces its orderly quantitative 

 effect by affecting one and the same reaction differently, for example, 

 by changing the rate and (or) the quantity of the end product, which 

 is probably the rule, other pleiotropic effects will follow the same 

 order only ff they are directly controlled by the same reaction. If 

 this is not so, the effects are secondary or are effects of interference. 

 The order of the other characters depends upon the developmental 

 system in which they appear. I once used the following model, which 

 may be varied in many ways ( Goldschmidt, 1932a ) : let us assume 

 A, B, C, D to be successive points in development at which. the alleles 

 mi, mo, ms, m4 act. One action determines the quantity of pigment in 

 a linear series proportional to the time of onset (A, B, C, D) of the 

 reaction determined by the alleles. Another consequence (direct, in- 

 direct, by interference ) of this reaction would be to affect the speed of 

 growth of another organ in a plus or minus direction. Let us now as- 

 sume — this is "the developmental system" of the case — that normal 

 growth of this part (which is independently determined) occurs in 

 the following seriation at the times A, B, C, D: growth in breadth — 

 in length — in length — in breadth. Now the alleles mi-m4 would pro- 

 duce the seriation of length-breadth index in favor of length: 



h H , that is, a seriation different from that of the pigment series. 



Similar models may be constructed also for specific biochemical events, 

 in which a single reaction progressing at a definite rate in different 

 alleles affects different processes of synthesis in an order not depend- 

 ing upon that rate, but upon the substrates available or not available 

 at different times for independent reasons. Thresholds, quantities of 

 specific enzymes, and so on could also come into play. A model of 

 this type was used by S. Emerson (1950) to explain the interrelations 

 of threonin-less mutants in Neurospora. Other possibilities of a simi- 

 lar type are mentioned by Haldane (1954): each allele might pro- 

 duce a series of enzymes with different absolute specificities, or hor- 

 mones stimulating growths in different parts of the body, in different 



