Imprinting: When Mom's Copy and Dad's Copy Differ

Two copies, but only one speaks

For most genes you carry two alleles — one from each parent — and both are switched on. Genomic imprinting breaks that symmetry. For an imprinted gene, the cell silences one copy based purely on which parent it came from, regardless of its DNA sequence. Some genes are expressed only from the father's copy; others only from the mother's.

The “parent-of-origin” tag is laid down by DNA methylation at a small control switch called an imprinting control region. Because the active copy is already chosen, an imprinted gene is unusually fragile: if the single working copy is lost or carries a germline mutation, there is no backup. That is why some imprinting disorders depend not just on which gene is affected, but on whether the change was inherited from the mother or the father.

An imprinted gene (active only from father):

  Father's copy:  [ ON  ]  -> protein made
  Mother's copy:  [ off ]  <- silenced by methylation

Lose the father's copy (deletion/mutation):
  Father's copy:  [ -- ]   (gone)
  Mother's copy:  [ off ]  <- still silenced -> NO working copy at all

Wiping the slate and writing it again

If marks were never erased, they would pile up forever. Instead, during the formation of gametes (eggs and sperm), the old imprints are stripped off and rewritten according to the sex of that parent. A gene that was on the “mother” setting in a man gets re-stamped to the “father” setting before it goes into his sperm, so his children receive a correctly labeled copy.

1. A fertilized egg inherits one imprinted copy from each parent, each already methylated for its source.
2. As the body grows, that mark is faithfully copied through every cell division.
3. When this person later makes gametes, the inherited imprints are erased.
4. Fresh imprints are written to match this person's own sex, ready to pass on.