154 



The Nucleus and Cytoplasm in Development 



of alleles such as those responsible for the 

 human iso-agglutinogens A and B and the 

 antigens themselves, as shown by the facts 

 that whenever the relevant allele is present 

 the antigen appears in every genetic back- 

 ground, and that in heterozygotes for the 

 alleles I* and F both antigens A and B are 

 produced simultaneously. 



It has also been assumed that enzymes, 

 which control a variety of biochemical re- 



different rates of gene-controlled, qualita- 

 tively identical reactions and has shown how 

 such a rate concept fits a great variety of 

 phenomena in developmental genetics. In 

 some cases, different rates of production of 

 pigments or of growth, controlled by differ- 

 ent alleles, have been demonstrated by direct 

 observation (Fig. 38.4 and B). These different 

 rates have been reported by Goldschmidt 

 ('27) for larval pigmentation in Lymantria 



Instors 



Ez: 



Sexual maturity Sexual maturity 



of normal males of dwarf males 



begins begins 



80 



30 40 50 

 Time in days 



Fig. 38. A, Pigmentation curves of seven different geographic races of the gypsy-moth Lymantria dispar 

 (after Goldschmidt, '27). B, Growth curves for a normal (GG) and a "dwarf" {gg) strain of Gammarus 

 chevreuxi (after Ford and Huxley, '29). 



actions in the bread mold Neurospora, are 

 under immediate gene control in the sense 

 that specific alleles impress enzymatic spec- 

 ificity on cellular constituents (Beadle, '45; 

 Bonner, '48; Horowitz, '50). Attempts to 

 prove this hypothesis have met with great 

 difficulties (Emerson, '50). 



The examples of gene-controlled antigens 

 and enzymes indicate that the primary prod- 

 ucts of the action of different alleles may 

 be qualitatively different. There are other 

 examples which suggest quantitative differ- 

 ence in allelic action. A series of multiple 

 alleles of a given gene visually affects the 

 same phenotypic trait, in a graded manner. 

 In Drosophila, for instance, different alleles 

 of the vestigial locus control the appearance 

 of a fully formed wing, of a slightly nicked 

 wing, a deeper notched one, a short vestig- 

 ial wing stump, and a wingless condition 

 (Mohr, '32). Goldschmidt ('38) has given 

 an interpretation of these cases in terms of 



dispar, and by Ford and Huxley ('29) for 

 rate of growth in Gammarus chevreuxi. 



Different rates of apparently identical 

 processes do not need to depend on different 

 rates of identical primary genie reactions. 

 Different alleles may control the production 

 of qualitatively different primary products 

 which in turn may influence identical re- 

 actions in quantitatively graded fashion 

 (Wright, '45a; Stern, '48). 



The theory that different alleles of a gene 

 cause their different developmental effects 

 by influencing the rate of gene-controlled 

 reactions has led Goldschmidt to the pre- 

 diction that the different phenotypes can be 

 produced pixrely environmentally in devel- 

 oping animals of identical genotype. External 

 agents, particularly temperature, should 

 change differentially the many gene-con- 

 trolled reactions and thus act, at appropriate 

 stages, essentially like shifts in reactions due 

 to different alleles. This expectation has been 



