Monozygotic twins share one genome, yet by adulthood certain genes differ in activity between them with no difference in DNA sequence. Which mechanism best accounts for the changed gene activity?

Core Concept

Epigenetic marks regulate access to genes without touching base pairs. Cytosine methylation in promoters usually dampens transcription, while histone tail modifications shift local chromatin toward silent or active states, and some marks persist through mitosis. Diet, environment, and random developmental events diverge between genetically identical people, layering different marks onto matching genomes. Those differing landscapes produce the observed activity differences.