THE OEIGIN OF VAEIATIONS IN SEXUAL AND 

 SEX-LIMITED CHARACTEKS 

 DR. CALVIN B. BRIDGES 

 Columbia Uxiversity 



In dealing with sex and its determination, attention lias 

 been most sharply focused upon forms with separate 

 sexes and upon the visible differences between the chro- 

 mosome groups of the two sexes. The result has been 

 that the formulation of sex-determination has remained 

 in terms of chromosomes, while the modern unit of deter- 

 mination is the gene ; and also the subject of sex has been 

 rather separated ot¥ from the main body of heredity. My 

 discussion will be largely a process of resolving chromo- 

 somes into component genes, and showing that the con- 

 ception of the nature and action of genes as gained from 

 the study of non-sexual characters is valid in interpreting 

 sex phenomena. 



The facts of mutation and of linkage have given us the 

 conception of a gene as a distinct chemical entity having 

 a definite location in a particular chromosome. Each 

 gene is essentially a factory, which is manufacturing a 

 characteristic set of chemical products that are delivered 

 to the common cytoplasm, and that produce development 

 through interaction with each other and with materials 

 from outside. But since the chemicals produced by the 

 different genes are different, some genes will have much 

 effect upon one character and little effect upon another, 

 so that a relatively small proportion of the genes will be 

 actively concerned in producing any given character. 

 Some of these genes tend to make the character more pro- 

 nounced, and others tend to make it less pronounced, so 

 that the grade of development actually realized by each 

 particular character will be determined by the equilibrium 

 between its modifying genes. The forms into which a 

 given character can be modified are in general quite di- 

 verse, but for the sake of simplicity we may call them all 

 plus or minus modifications. If the effectiveness of a 

 given plus or minus modifier is changed by mutation, the 

 grade of the character will shift correspondingly. 



We can conceive of the evolution of the sexual 

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