DIFFERENT GENETIC POSSIBILITIES 

 WITHIN THE BALANCE THEORY 



The general facts of genetics make us expect that within the general 

 features of the F/M balance, variants in detail will be found. Thus 

 we know that in Drosophila or man, sex-linked loci are numerous, 

 while they are very rare in rodents. Further, the Y-chromosome in 

 man and fishes has a considerable pairing segment which can cross 

 over with the X-chromosome, while Drosophila does not have much 

 of it. Also, certain traits of similar type may be inherited in one species 

 as simple Mendelian traits; in another one, on a multifactorial basis. 

 Thus we cannot be surprised to find variations of the same theme for 

 sex determination also. 



A. VARIANTS IN REGARD TO SEX DETERMINERS 



Such variants may first be found in regard to the sex determiners 

 themselves. I had originally assumed that F and M in Lymantria are 

 single loci, since no facts were found which could be interpreted as 

 pointing to a group of Hnked loci. (A few extremely rare exceptional 

 intersexes where none were expected were interpreted by others as 

 results of crossing over, but without any proof. ) However, Dobzhansky 

 and Schultz (1934) showed that the X-chromosome in Drosophila 

 does not contain a single F locus, but has female action in many 

 sections of its length, which was interpreted as a set of multiple sex 

 factors. The details do not concern us here. Actually, Pipkin (1942) 

 found facts which do not agree with the simple conclusion. There are 

 modifying actions of small fragments which can be distinguished from 

 primary sex determination. The latter was established only for sufiS- 

 ciently large segments at either end of the X, without any intimation 

 as to what acted in these segments. "Multiple sex factors," in this case, 

 does not mean much more than a statement that no single "F gene" 

 was found. As I have already stated {I 2 C d ee), I feel that we 

 are entitled to assign the F function here to the intercalary hetero- 

 chromatin. It would, however, be wrong to conclude that, therefore, in 

 Lymantria also the X must contain multiple sex factors or that all sex 

 chromosomes must harbor such sets. It is possible that in individual 

 432 



