Monozygotic twins, although genetically very similar, are not genetically exactly the same.
The DNA in white blood cells of 66 pairs of monozygotic twins was analyzed for 506,786 single nucleotide polymorphisms known to occur in human populations.
Polymorphisms appeared in 2 of the 33 million comparisons, leading the researchers to extrapolate that the blood cells of monozygotic twins may have on the order of one DNA-sequence difference for every 1.2 x 10 The mutations producing the differences detected in this study would have occurred during embryonic cell-division (after the point of fertilization).
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However, it is only the mother who has any effect on the chances of having such twins; there is no known mechanism for a father to cause the release of more than one ovum.
Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.
In fraternal twins, each twin is fertilized by its own sperm cell.
In Latin America, South Asia, and Southeast Asia, the lowest rates are found; only 6 to 9 twin sets per 1,000 live births.
Another cause of difference between monozygotic twins is epigenetic modification, caused by differing environmental influences throughout their lives.
Epigenetics refers to the level of activity of any particular gene.
A gene may become switched on, switched off, or could become partially switched on or off in an individual.
This epigenetic modification is triggered by environmental events.
However, dizygotic twins may also look very different from each other.