90 Nature of the Genetic Material 



much still left to be cleared before our hypothesis may be considered 

 as proved. 



ee. Heterochromatin and the genetics of sex 



One more group of facts is strongly suggestive and has been 

 used (Goldschmidt, 1949c) in presenting the hypothesis that hetero- 

 chromatic heredity is involved. Some discussion by Pontecorvo ( 1943 ) 

 could be considered as foreshadowing this idea. One type of heredity, 

 the heredity of sex, is completely different from all others. A single 

 hereditary difference, working, as a first approximation, like a Men- 

 delian backcross, determines maximally a difference affecting the 

 entire organism, actually every single cell. This is the most general- 

 ized genetic action known; in many instances it produces phenotypic 

 differences of the entire body or of one or many individual parts, 

 which, purely quantitatively, are of the order of specific, generic, 

 and still higher differences. If, for example, we were to consider the 

 highly different genital armatures as two individual organisms, these 

 would appear as different as if they belonged to different taxonomic 

 classes. Furthermore, the genetic differences between the sexes in 

 higher diploid organisms like insects exhibit a type otherwise un- 

 known in genetics. It is well known what this is: (1) the differentia- 

 tion of a special pair of chromosomes, the X-chromosomes, which 

 play the decisive role by a kind of backcross mechanism which takes 

 care that half of the fertilized eggs normally receive two, and the 

 other haff one X-chromosome; (2) the establishment of genetic fac- 

 tors for the control of one of the sexes (e.g., the male in Lymantria, 

 the female in Drosophila) located within the X-chromosomes; (3) 

 the provision of genetic factors for the control of the other sex out- 

 side the X-chromosomes (in some the Y-chromosome or autosomes or 

 both); (4) the control of the sexual alternative, that is, of the entire 

 female or male phenotype with all its differences by a simple quanti- 

 tative ratio or balance: one dose X-chromosomal determiners in- 

 ferior, two doses superior, to the identical (homozygous) action 

 of the determiners outside the X-chromosomes. This genetic mecha- 

 nism, which works at least in all diploid, genuinely dioecious organ- 

 isms, is an old specific feature different from the usual interplay of 

 Mendelizing standard loci. It is at once suggestive that the seat of 

 these genetic factors (I purposely use this general terminology) are 

 the two chromosomes which are known to be the richest in hetero- 

 chromatin and to exhibit most prominently all the characteristic 

 features of heteropyknosis and allocycly, so well known to cytology. 



