Genomic Imprinting & Uniparental Disomy
Genomic Imprinting
- Genomic imprinting refers to parent-specific gene activation: gene expression that is limited to either expression of the gene from the father or the mother (rather than both genes being active, like normally takes place).
- In imprinted genes, only one of the two copies (paternal or maternal) is actively expressed (normally both copies are).
Uniparental Disomy
- Uniparental disomy occurs when the child receives two copies of a chromosome (or a fraction of the chromosome) from one parent and none from the other.
- In Prader-Willi syndrome and Angelman syndrome, deletions take place within the same region of chromosome 15 but the phenotypic expression depends on whether the gene that is abnormal is from the mother or father because certain genes within that region are only paternally-expressed whereas others are only maternally-expressed (they are imprinted).
- When the paternal chromosome 15 bears the deletions, the child manifests with Prader Willi syndrome: Prader Willi syndrome results from an absence of genes on 15q11- q13 that are paternally-expressed.
- When the maternal chromosome 15 bears the deletions, the child manifests with Angelman syndrome: Angelman syndrome results from an absence of genes on 15q11- q13 that are maternally-expressed.