The epigenetics of nucleolar dominance

Epigenetic phenomena are heritable, alternative stales of gene activity tha t are not explained by mutation, changes in gene sequence or normal develop mental regulation. Among the earliest examples was nucleolar dominance, a c ommon phenomenon in interspecific hybrids in which only ribosomal RNA (rRNA ) genes inherited from one parent are transcribed. Only active rRNA genes i nitiate 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 seq uence differences that affect transcription factor affinities. Other eviden ce suggests that chromosomal context is more important than rRNA gene seque nces, implying control on a larger scale.