We all know identical twins. Despite significant advances in most areas of science, it is not known exactly what causes identical (identical or MZ) twins. MZ twins randomly appear in families at the rate of about four per 1000 births. Scientists have investigated genetic predispositions and environmental pressures as the cause of MZ twins, but the studies have yet to pinpoint what leads to MZ twins.
In comparison, the other type of twins, known as non-identical or dizygoti (DZ) twins, appear to run strongly within families, suggesting a genetic component. Researchers know that the processes that lead to the twin types are markedly different. DZ twins arise from double ovulation, while identical twins arise from a single embryo that divides into two parts.
It is not known why the individual embryo divides into two parts at this point. Researchers have made a discovery that shows that epigenetic information in chromosomes differs between identical twins and other humans. The differences are not in the DNA code itself, but in the associated chemical markings. The study, carried out by researchers from Boomsma at Vrije Universiteit Amsterdam (VU), used twin registries from the Netherlands, Great Britain, Finland and Australia.
The researchers measured the degree of methylation at over 400,000 DNA sites in 6000 pairs of twins. They discovered 834 sites in DNA where the degrees of methylation in identical twins differed from those in non-twins. Researchers say this is a big discovery, and the study marks the first time a biological marker for twins has been discovered in humans.
Project scientists say the explanation for identical twins appears to be not in the genome, but in the epigenome. The epigenome is described as a control element that determines how genes are coordinated and how strongly they are expressed around the building blocks of DNA. Researchers believe the study could lead to a better understanding of congenital abnormalities that are more common in MZ twins.
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