Bivalent chromatin is chromatin

Chromatin

Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...

 that contains both activating and repressing epigenetic modifications in the same area. Activating chromatin modifications increase the accessibility of the chromatin to RNA polymerase

RNA polymerase

RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...

, where repressing modifications decrease the accessibility to RNA polymerase. Usually, these do not both occur at the same location, as it seems they would have countering effects; however in bivalent chromatin, they are both present. The repressing modifications usually take precedence, causing the gene to become inactivated. Once the repressing modifications are removed however, the activating modifications attract transcription

Transcription (genetics)

Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...

 machinery, and the gene becomes activated.

Bivalent chromatin is hypothesized to serve a role in genetic imprinting

Imprinting (genetics)

Genomic imprinting is a genetic phenomenon by which certain genes are expressed in a parent-of-origin-specific manner. It is an inheritance process independent of the classical Mendelian inheritance. Imprinted alleles are silenced such that the genes are either expressed only from the non-imprinted...

and in development.