l88 AN INTRODUCTION TO MODERN GENETICS 



brium. This is rarely apparent in genetic experiments, but is much 

 more clear in experimental embryology, where an experiment usually 

 consists in carrying the point representing the state of a process into 

 some unnatural place in the landscape defined by the genotype, and 

 from this place the process may run down into the normal valley (i.e. 

 equiUbrium is restored, regulation occurs). The equilibrium aspect of 

 the valley only appears in genetics if by chance the valley bottom 

 becomes flat, i.e. there are several equally probable values for the 

 process. Then the gene which causes the flattening is observed to lead 

 to variable results. Timofeeff'-Ressovsky^ has described a good case 

 where the only effect of a gene seems to be to flatten the bottom so 

 that the phenotype varies equally on both sides of the normal; flies of 

 D. funehris with the gene may develop either more or fewer bristles 

 than the wild type. Another similar gene affecting wing-venation is 

 illustrated in Fig. 88. 



7. The Variability of Gene Effects 



The effects produced by a gene vary in dependence on both the rest 

 of the genotype and the environment. Timofeeff"-Ressovsky2 has pro- 

 posed discussing these variations in terms of the concepts of expressivity, 

 penetrance, and specificity. 



(a) Expressivity. — The expressivity is a measure of the amount of 

 effect shown by the gene. If we are dealing with a quantitative char- 

 acter it can be measured by the quantity of substance produced, either 

 in the appropriate real units or in terms of some arbitrary unit. We 

 have already seen that the expressivity of a gene depends on the rest of 

 the genotype, since modifying genes affect the degree or quantity of 

 the effect produced. It also depends on the environment in many cases. 

 Thus if we have Bar in a given genotypic milieu in Drosophila, the 

 number of facets formed depends on the temperature during the 

 sensitive period (p. 184). 



The dependence of expressivity on environment is particularly 

 important in connection with the problem of determining how far the 

 differences between two organisms are hereditary and how far they are 

 caused by different external conditions during development. If we have 

 two Bar flies reared at different temperatures, the ratio of the number 

 of facets in the two types gives a measure of the effect of the environ- 

 ment on the expressivity of Bar; and if we have Bar and not-Bar flies 



^ Timofeeff-Ressovsky and Timofeeff-Ressovsky 1934. 

 - Timofeeff-Ressovsky 193 1. 



