The epigenetics of nucleolar dominance.

Epigenetic phenomena are heritable, alternative states of gene activity that are not explained by mutation, changes in gene sequence or normal developmental regulation. Among the earliest examples was nucleolar dominance, a common phenomenon in interspecific hybrids in which only ribosomal RNA (rRNA) genes inherited from one parent are transcribed. Only active rRNA genes initiate formation of a nucleolus, hence the name for the phenomenon. As in other epigenetic phenomena, chromatin modifications enforce selective gene silencing in nucleolar dominance. However, the mechanisms that discriminate between parental sets of rRNA genes are unclear. Possibilities include sequence differences that affect transcription factor affinities. Other evidence suggests that chromosomal context is more important than rRNA gene sequences, implying control on a larger scale.