In Mendelian inheritance we usually inherit two working copies of a gene; one from the mother and one from the father. But with certain genes, we receive only one working copy of that gene from either the mother or father. Who we receive the working copy from depends entirely on the gene itself. This means that one copy is epigenetically silenced and the other is active.
Silencing of a gene happens during the development of the egg or the sperm. Imprinted genes occur in clusters, the cluster is silenced by the enzymatic addition of a methyl group to the cytosine of a CpG dinucleotide. within the cluster which decreases transcription of nearby genes. This is performed by methylases and is a control method used on many genes, not just those to be imprinted.
Imprinted genes remain methylated during fertilisation. Imprinting is erased early in germ development and re-established about the time of birth. This makes sure the genes are inactivated due to parent inheritance. Imprinting has no effect on genes inherited from grandparents.
Developmental problems and disease occur when the active gene is a mutant or the gene to be silenced is not silenced. Other problems occur if the cells receive all or part of the chromosomes from a single parent. So you could end up with a cell with two imprinted inactive copies or two active copies leading to over or underexpression.
- ↑ Medical Genetics: Lecture Notes (2006) Bradley J et al (3rd Edition) P56