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Imprinting, required for normal development, is unique to mammals and flowering plants. In mammals, about 1% of genes are imprinted. For most genes, humans inherit two working copies: one from each biological parent. But with imprinted genes, you inherit only one working copy. Depending on the gene, either the copy from mom or the copy from dad is epigenetically silenced. Silencing usually happens through the addition of methyl groups during egg or sperm formation. Imprinted genes are especially sensitive to environmental signals. Because imprinted genes have only a single active copy and no back-up, any epigenetic changes or "epimutations" will have a greater impact on gene expression. Environmental signals can also affect the imprinting process itself. Imprinting happens during egg and sperm formation, when epigenetic tags are added to silence specific genes. Diet, hormones and toxins can all affect this process, impacting the expression of genes in the next generation. (http://learn.genetics.utah.edu/content/epigenetics/imprinting/)