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X Chromosome differences in idential twins

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  • X Chromosome differences in idential twins

    There was a recent scientific paper on identical twins not having identical DNA, specifically that some identical twin pairs have up to 100 genetic differences (non-matching positions in their DNA). Given I am a father of identical twins and they each have had an autosomal analysis performed, I decided to compare their data. About 99.38% of their data matched exactly, and of the remainder, most of the differences were due to a failure to obtain a value for one or the other at a position. Only 503 of 722,126 positions (~0.07%) differed after missing data were accounted for.

    What was curious was that nearly 89% (446) of these real differences between sons' DNA were on the X chromosome! Can anyone explain this high percentage of differences being on X?

    Further, only 8 of the 446 (<2%) were cases where only one letter of a pair differed from the other son's pair (for example, CT vs TT ). All of the rest had both letters in a son matching and both letters differing from the other son (for example TT vs GG). No differences were of the form CT vs AG or CT vs AA (both letters different). I'll note that their mother has about 27% positions with non-matching letters.

    I have absolutely no explanation for these seemingly odd results. Again I ask, can anyone explain why this happened?