In an experiment, it was found that newborn rats who were genetically predisposed to be skittish, nervous, and high-strung would develop into calm, exploratory, and stress-resistant adult rats when raised by genetically unrelated, attentive mothers. Although the rats' DNA did not change, the chemicals that controlled their gene expression did change, a phenomenon called: a) epigenetic change. b) allele variation. c) recessive gene mutation. d) dominant gene mutation.