CYTOGENETIC CORRELATIONS AND CROSSING OVER 95 



Chromosomes contain regions which stain intensely with basic dyes 

 and have been called heter achromatic, in contrast with faintly staining 

 areas, referred to as euchromatic. A popular interpretation of this 

 staining difference has been in terms of a differential condensation or 

 coiling of the chromosome backbone. The fact that chromosomes do 

 coil is evident at the level of the light microscope, as seen, for example, 

 in Tradcscantia. In the Drosophila salivary chromosomes, hetero- 

 chromatic regions are seen near the centromeres of chromosomes II, 

 III, and X, which are coalesced forming the chromoccnter; and the 

 entire Y chromosome is considered heterochromatic. Bits of hetero- 

 chromatic material are scattered throughout the Drosophila genome as 

 well as concentrated in the chromocenter. The same is probably true 

 in most organisms. 



Two features have been noted in the behavior of heterochromatic 

 regions: (1) Genes are rarely found there, and (2) the expression of 

 dominant genes from euchromatin is often blocked when such genes are 

 moved into heterochromatic regions by chromosome rearrangements after 

 X-irradiation in Drosophila. This phenomenon, called position effect, 

 was the first evidence of gene interaction at the chromosome level, 

 an exceedingly important problem to be discussed in Chapter IL In 

 many organisms B chromosomes which are heterochromatic are present 

 in some strains and not in others. They are distinguished by the fact 

 that no change in phenotype is evident when their number is increased 

 or diminished by appropriate crosses. In many species, the sex chromo- 

 somes are heterochromatic. In Drosophila very few genes have been 

 located on the Y chromosome, yet it plays a decisive role in the forma- 

 tion of normal sperm. 



There are no generally accepted criteria for the identification of 

 heterochromatin, and it is possible that a number of differently func- 

 tioning regions have been grouped under this heading. Those that 

 exert position effect might constitute one group, but this criterion would 

 be useful only in organisms such as Drosophila in which position effect 

 can be readily studied. Recent evidence that heterochromatic regions 

 undergo duplication at a different time in the mitotic cycle than do 

 euchromatic regions might provide another criterion if preliminary ob- 

 servations are confirmed in other organisms. One may sum up the 

 present situation as follows: neither the chemical, the structural, nor 

 the functional properties of heterochromatin have been established, but 

 nonetheless there is a widespread feeling among cytologists and geneti- 

 cists that heterochromatic material plays some biologically distinct and 

 significant role. 



