Heterochromatin and euchromatin

In 1928 Emil Heitz observed that certain parts
of the chromosomes of a moss (Pellia epiphylla)
remain thickened and deeply stained during interphase,
as chromosomes otherwise do only
during mitosis. He named these structures heterochromatin,
as opposed to euchromatin,
which becomes invisible during late telophase
and subsequent interphase. Functionally, heterochromatin
is defined as a region in which few
or no active genes lie and in which repetitive
DNA sequences occur. When active genes become
located close to the heterochromatin,
they usually become inactivated