Pikaard C S
Biology Dept, Washington University, Campus Box 1137, One Brookings Drive, St Louis, MO 63130, USA.
Trends Genet. 2000 Nov;16(11):495-500. doi: 10.1016/s0168-9525(00)02113-2.
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.
表观遗传现象是可遗传的基因活性替代状态，无法用突变、基因序列变化或正常发育调控来解释。最早的例子之一是核仁显性，这是种间杂交中的常见现象，其中只有从一个亲本遗传而来的核糖体RNA（rRNA）基因被转录。只有活跃的rRNA基因启动核仁的形成，因此得名该现象。与其他表观遗传现象一样，染色质修饰在核仁显性中强制选择性基因沉默。然而，区分亲本rRNA基因集的机制尚不清楚。可能性包括影响转录因子亲和力的序列差异。其他证据表明染色体背景比rRNA基因序列更重要，这意味着在更大规模上的控制。
