These images are created by probes to sequences located throughout the length of the chromosome and these probes are tagged with different fluorescent groups in known proportions, different for each chromosome. Data are collected by a detection mounted on a fluorescent microscope and computer image reconstruction is then used to create false color images of the chromosomes. Electron micrograph of a thin section of a bone-marrow stem cell. Areas of the nucleus that remain darkly staining when the condensed chromosomes have returned to their decondensed state in interphase are called heterochromatin. The decondensed, light staining regions are called euchromatin.
Most transcription occurs in the euchromatin regions and in the nucleus.
Here we see heterochromatin located near the nucleolus and in association with the nucleoplasmic face of the nuclear membrane. The answer seems to be that it is somewhat heterogeneous. For example, it has been studied in Drosophila polytene chromosomes where it is often but not exclusively found at the centromeres and telomeres. Most of the DNA in heterochromatin is simple-sequence or repetitive DNA, but some genes, probably mostly inactive, are located there as well. Evidence for these structures have been most convincing collected from yeast genetic studies.