l80 AN INTRODUCTION TO MODERN GENETICS 



which different starches are laid down very soon after the segregation 

 of the genes. In grains containing waxy, the amylase concerned with 

 the synthesis of the starch is different from the normal enzyme, and the 

 starch formed stains red-brown instead of blue with iodine. The 

 abnormal type of starch is correlated with a slower pollen tube growth, 

 and it is possible that there are many other biochemical differences yet 

 to be detected in the numerous cases known in which pollen tube 

 growth is directly affected by the particular allelomorph contained in 

 the gamete. No attempts have apparently been made to detect enzymes 

 in segregating animal sperm. 



3. Time-effect and Dose-effect Curves 



In the examples mentioned in the last section we have attempted to 

 describe the course of the developmental reactions by which a certain 

 substance is produced. We might summarize the reactions in a given 

 case by plotting the quantity of substance present against the time. The 

 curve which would be obtained might be called the time-effect curve 

 of the gene under investigation. We shall in this section try to generalize 

 this idea so as to make the time-effect curve of a gene summarize 

 all the information which we have about the developmental action of 

 the gene. 



In the first place, we must inquire into the relations between the 

 time-effect curve and the dose-effect curves of the same gene. The 

 dose-effect curves which were discussed earlier (p. 164) were obtained 

 by plotting the dose of the gene against the final effect produced in the 

 adult organisms. From a developmental point of view, the final effect 

 of a gene in the adult must either be an asymptote to the time-effect 

 curve, when development slows off gradually as maturity is attained, or 

 in some animals it may be an end-value reached when development is 

 suddenly brought to an end by a metamorphosis. In either case, the 

 end-value of the time-effect curve is the same as the value plotted for 

 that gene on the dose-effect curve. If we have a set of allelomorphs, the 

 dose-effect curve is in fact merely a summary of the end-values of the 

 separate time-effect curves. 



The importance of this point is that it shows that certain conclusions 

 about dose-effect curves also apply to time-effect curves. For instance, 

 we have rather Httle detailed information about the dependence of the 

 time-effect curves on the genotypic milieu, although Ford and Huxley 

 have described the effect of some modifying factors on the time-effect 

 curve of pigment formation in the eyes of Gammarus and a few other 



