Nucleolar dominance and silencing of transcription

Nucleolar dominance is a phenomenon in plant and animal hybrids whereby one parental set of ribosomal RNA (rRNA) genes is transcribed, but the hundred s of rRNA genes inherited from the other parent are silent, The phenomenon gets it name because only transcriptionally active rRNA genes give rise to a nucleolus, the site of ribosome assembly. Nucleolar dominance provided th e first clear example of DNA methylation and histone deacetylation acting i n partnership in a gene-silencing pathway. However, the sites of chromatin modification and the ways in which one set of rRNA genes are targeted for r epression remain unclear. Another unresolved question is whether the units of regulation are the individual rRNA genes or the multi-megabase chromosom al domains that encompass the rRNA gene clusters.