202 Outline of Genetics 



One might then assume that in organisms where the unisexual 

 condition has existed for some time back in phylogeny, a definite 

 sex chromosome mechanism has been established. This mecha- 

 nism insures (normally) the production of two types of individuals 

 in equal numbers, those which are preponderantly males and those 

 which are preponderantly females. The grade of sex does not 

 appear to fluctuate in response to varying environmental influences, 

 since these influences are relatively insignificant in such cases. 

 In other organisms, however, which have more recently been 

 evolved from bisexual ancestors, a regular sex chromosome mecha- 

 nism has not yet been perfected. The appropriate machinery is 

 not yet at work to produce individuals which are preponderantly 

 male and individuals which are preponderantly female in equal 

 numbers. Instead, sex is being influenced by numerous factors 

 which are distributed sporadically rather than in organized groups 

 as in the fruit fly. The net effect of these factor influences is 

 commonly near to the point of equilibrium, so that the organism 

 is more responsive to environmental influences on sex grade. 

 Under such conditions the sex grade and the sex ratio may be 

 susceptible to a certain amount of artificial control through manip- 

 ulation of the effective environmental influences. 



Finally, this work of Bridges' casts a new light upon the whole 

 subject of unit characters. Careful investigation of flies which are 

 triploid with respect to one or more chromosome sets, leads 

 Bridges to draw the same general conclusions with regard to 

 other so-called unit characters that he drew with regard to the 

 character of sex. Many characters have their degree of develop- 

 ment influenced, not merely by the presence or absence of certain 

 single genes, but by the net effect of the influences of numerous 

 genes. It is true that there is commonly one gene that exerts a 

 greater influence on the character in question than do any other 

 genes, and it is quite common that all the other genes may be 

 constant in their presence or absence, so that only the effects of 

 the one gene are noticeable, and we identify it as " the determiner" 

 of the character in question. In such cases the equiUbrium of 

 •opposing influences is normaUy being affected to a perceptible 

 •degree only by the presence or absence of a single gene. Abnormal 

 situations, however, may arise, as the result of non-disjunction of 



