Definition
Genomic imprinting is a genetic phenomenon by which certain genes are silenced on one of the 2 alleles and are expressed in a parent of origin-specific manner. 
The genes in these regions are haploid, only one allele works.
The male and female genomes in mammals were not interchangeble, even if they possessed identical DNA sequences.
Epigenetic factors help establish and maintain genomic imprinting by controlling how tightly the cromatin is coiled, despite a lack of change in the DNA sequence.
It must be reset in each new generation to ensure appropriate gene activity.
In primordial germ cells, DNA copying is followed by erasing of old imprints and the restablishment of newly uniform imprints that reflect the offspring's own sex.
The genome imprinting is regulated by epigenetic mechanisms .
Methilation is the epigenetic basis for imprinting:
- A DNA methyltransferase connects a methyl group (CH3-) to a cytosine in DNA
- Double helix shape modification
- Cromatin condensation
- No binding of accessory protein
- Gene activity regulation
Methylation can be maintained even if the DNA is copied.
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Imprinted and more Equal
