Constitutive heterochromatid is densely packed chromatin (different from functional rather diffused euchromatin) which is non-function, access of transcription factors is prohabited in those regions. So, I would like to know, why they are here? Are they just another group of junk DNA pieces (will any phenotypical change be observed if mutation is directed into such a region?), just like introns? Or they are once active in the embryonic development process or the ontogeny of an individual?

I'd be tempted to speculate that they play roles structural integrity as well in chromosome segragation-IIRC thats what the alpha-satellites are involved in anyway. I'm not sure of the exact role they do play though, if any, or it may be that different regions of the chromsome have functions whereas others do not and in fact contain many transposable elements. In this latter case it could further be speculated that the constituitive heterochromatin is essentially protecting the genome from potentially detrimental transpon events. What specific marks or sequence induces chromatin condensation in these particular cases is probably somewhat similar to those involved in other forms of heterochromatin, i.e. hypermethylation of CpGs and extensive deacetylation of histone residues.

 

Someone more informed may be able to shed a better light on things though.