Final answer:
Facultative heterochromatin refers to the regions of chromatin that differ in compaction among various cell types and can be altered for gene regulation, whereas euchromatin is typically less condensed and actively transcribed.
Step-by-step explanation:
The regions of heterochromatin that vary among different cell types are called facultative heterochromatin. Constitutive heterochromatin, on the other hand, is uniform across all cell types and typically includes regions such as centromeres and telomeres. Facultative heterochromatin varies because it can be condensed or decondensed depending on the cell type and the developmental stage, thus influencing gene expression.
Euchromatin is the less condensed form of chromatin and typically contains active genes that are being transcribed. During interphase, eukaryotic chromosomes have both euchromatin and heterochromatin regions, suggesting a dynamic structure that can change based on cellular needs. When genes in facultative heterochromatin are turned 'on' or 'off', the structure toggles between more open euchromatin and tightly packed heterochromatin.