Genie and Non-genlc Parts of the Chromosome 91 



We might summarize (details will be analyzed in a later chap- 

 ter) the genetic facts which seem to fall in line with the special 

 cytology of the sex chromosomes. In Drosophila a very thorough 

 search for the genetic sex factors in the X-chromosomes could be 

 made since Dobzhansky and Schultz (1934) introduced the method 

 of adding or subtracting broken fragments of the X to the normal 2X 

 of triploid intersexes and measuring the feminizing or masculinizing 

 effect by the grade of intersexuality. The most detailed work was 

 done by S. Bedichek Pipkin (1940, 1942, 1947). The first unequivocal 

 result is that no single female sex gene could be located. In a general 

 way the feminizing eflcect of extra sections is proportional to the 

 size of the fragment. From this the conclusion was drawn that multi- 

 ple female sex "genes" are spread over the X-chromosome. But there 

 are some significant details. The most important one is (Pipkin) that 

 the part of the X-chromosome to the left of section 17 has more 

 feminizing influence than that to the right. Actually, almost all 

 intercalary heterochromatin is found in that left section with rather 

 equal distribution. Though no exact localization is possible in these 

 experiments, there are no facts which would be opposed to the loca- 

 tion of the female determining action in the series of intercalary 

 heterochromatic blocks. With this, however, we do not mean multiple 

 "genes" like other Mendelizing loci. We mean general actions of 

 heterochromatin blocks, without any need for special genes located 

 therein, actions upon generalized processes of early development 

 that decide the growth properties of the cells, which after all make 

 up the morphological sex differences. The genetics of sex determina- 

 tion, the balance and detailed genetic setup, are not under discussion 

 here, but solely the facts pointing to heterochromatin. 



A number of detailed facts agree with such a conclusion, which 

 would of course have to be applied also to the niale determining 

 sections outside the X-chromosome (in Drosophila). In Lymantria, 

 where the female determiners are located outside the X-chromosomes, 

 they were found in the Y-chromosome and thus inherited from mother 

 to daughter. Cytologically the Y could not be distinguished, and 

 therefore its heterochromaticity is not known. Also in Bombyx mori 

 the Y-chromosome contains the female factors (Tazima, 1943), in 

 this case very powerful ones which are not suppressed by a multiplic- 

 ity of male X factors. In addition to the main female determiners in 

 the Y-chromosome, autosomal modifiers are known, but again no 

 information about heterochromatin in the autosomes is available 

 except the fact of prophasic association mentioned above (Gold- 



