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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).

However, dizygotic twins may also look very different from each other.If they occur early in fetal development, they will be present in a very large proportion of body cells.Another cause of difference between monozygotic twins is epigenetic modification, caused by differing environmental influences throughout their lives.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.Comparison of zygote development in monozygotic and dizygotic twins.

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