<P> A high - throughput study, which denotes technology that looks at extensive genetic markers, focused on epigenetic differences between monozygotic twins to compare global and locus - specific changes in DNA methylation and histone modifications in a sample of 40 monozygotic twin pairs . In this case, only healthy twin pairs were studied, but a wide range of ages was represented, between 3 and 74 years . One of the major conclusions from this study was that there is an age - dependent accumulation of epigenetic differences between the two siblings of twin pairs . This accumulation suggests the existence of epigenetic "drift". Epigenetic drift is the term given to epigenetic modifications as they occur as a direct function with age . While age is a known risk factor for many diseases, age - related methylation has been found to occur differentially at specific sites along the genome . Over time, this can result in measurable differences between biological and chronological age . Epigenetic changes have been found to be reflective of lifestyle and may act as functional biomarkers of disease before clinical threshold is reached . </P> <P> A more recent study, where 114 monozygotic twins and 80 dizygotic twins were analyzed for the DNA methylation status of around 6000 unique genomic regions, concluded that epigenetic similarity at the time of blastocyst splitting may also contribute to phenotypic similarities in monozygotic co-twins . This supports the notion that microenvironment at early stages of embryonic development can be quite important for the establishment of epigenetic marks . Congenital genetic disease is well understood and it is clear that epigenetics can play a role, for example, in the case of Angelman syndrome and Prader - Willi syndrome . These are normal genetic diseases caused by gene deletions or inactivation of the genes, but are unusually common because individuals are essentially hemizygous because of genomic imprinting, and therefore a single gene knock out is sufficient to cause the disease, where most cases would require both copies to be knocked out . </P> <P> Some human disorders are associated with genomic imprinting, a phenomenon in mammals where the father and mother contribute different epigenetic patterns for specific genomic loci in their germ cells . The best - known case of imprinting in human disorders is that of Angelman syndrome and Prader - Willi syndrome--both can be produced by the same genetic mutation, chromosome 15q partial deletion, and the particular syndrome that will develop depends on whether the mutation is inherited from the child's mother or from their father . This is due to the presence of genomic imprinting in the region . Beckwith - Wiedemann syndrome is also associated with genomic imprinting, often caused by abnormalities in maternal genomic imprinting of a region on chromosome 11 . </P> <P> Rett syndrome is underlain by mutations in the MECP2 gene despite no large - scale changes in expression of MeCP2 being found in microarray analyses . BDNF is downregulated in the MECP2 mutant resulting in Rett syndrome . </P>

What is the term for an alternate state of a gene