Certain regions of chromosomes, particularly those proximal to centromeres are constant, and are called constitutive heterochromatic regions serving as chromosome markers. There are other heterochromatic regions called facultative heterochromatic and represented by whole sex chromosomes which become heterochromatic only at certain stages. For instance in female humans, one X-chromosome is inactivated or becomes heterochromatic only facultatively. Similarly in plants accessory chromosomes are heterochromatic. In plants also, among dioecious genera like Melandrium and Rumex, one or both sex chromosomes may undergo partial or complete heterochromatinization.
It is established that DNA replicates in heterochromatic region at a time different than that in euchromatic regions. It is also established that genes in heterochromatic region are inactive, but the earlier belief that no genes are found in heterochromatic regions is not correct because, genes could be located in heterochromatic regions in several cases like Drosophila and tomato. The genes in heterochromatic region perhaps become active for a short period. Y chromosome is another example of heterochromatic chromosomes having inactive genes in several dioecious plants and animals.