Board Thread:General Discussion/@comment-25301131-20150310010922/@comment-14512598-20150815183144

"Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if a pair of monozygotic twins reproduces with another pair or with the same person), rather than first cousins. Identical twins do not have the same fingerprints, due to the fact that even in a small space inside the womb, people have contact with different parts of this environment, which gives small variations in the same digital, making them unique. [23]   Monozygotic twins always have different   phenotypes. Normally due to an environmental factor or the deactivation of different X chromosomes in female monozygotic twins, and in some extremely rare cases, due to   aneuploidy, twins may express different sexual   phenotypes, normally from an XXY   Klinefelter syndrome  zygote splitting unevenly. [24] [25] [26]

<p style="margin-top:0.5em;margin-bottom:0.5em;line-height:22.3999996185303px;color:rgb(37,37,37);font-family:sans-serif;font-size:14px;">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<sup style="line-height:1;font-size:11.1999998092651px;">7  nucleotides, which would imply hundreds of differences across the entire genome.<sup class="reference" id="cite_ref-J_Med_Genet_27-0" style="line-height:1;font-size:11.1999998092651px;display:inline-block;">[27]  The mutations producing the differences detected in this study would have occurred during embryonic cell-division (after the point of fertilization). If they occur early in fetal development, they will be present in a very large proportion of body cells.

<p style="margin-top:0.5em;margin-bottom:0.5em;line-height:22.3999996185303px;color:rgb(37,37,37);font-family:sans-serif;font-size:14px;">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. Monozygotic twins can have markedly different epigenetic profiles. A study of 80 pairs of monozygotic twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference.<sup class="reference" id="cite_ref-28" style="line-height:1;font-size:11.1999998092651px;display:inline-block;">[28]  However, certain characteristics become more alike as twins age, such as IQ and personality. This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behavior.<sup class="reference" id="cite_ref-29" style="line-height:1;font-size:11.1999998092651px;display:inline-block;">[29] <sup class="reference" id="cite_ref-30" style="line-height:1;font-size:11.1999998092651px;display:inline-block;">[30] <sup class="reference" id="cite_ref-31" style="line-height:1;font-size:11.1999998092651px;display:inline-block;">[31]

<p style="margin-top:0.5em;margin-bottom:0.5em;color:rgb(37,37,37);font-family:sans-serif;"> A 1981 study of a deceased triploid XXX twin fetus without a heart showed that although its fetal development suggested that it was an identical twin, as it shared a placenta with its healthy twin, tests revealed that it was probably a polar body twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning.<sup class="reference" id="cite_ref-32" style="line-height:1;font-size:11.1999998092651px;display:inline-block;">[32]  In 2003, a study argued that many cases of triploidity arise from semi-identical twinning.<sup class="reference" id="cite_ref-33" style="line-height:1;font-size:11.1999998092651px;display:inline-block;">[33]  In 2007, a study reported a case of a pair of living twins, one intersex and one aphenotypical male. The twins were both found to be chimeras and to share all of their maternal DNA but only half of their father's DNA. The exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning. "

<p style="margin-top:0.5em;margin-bottom:0.5em;color:rgb(37,37,37);font-family:sans-serif;"><span style="color:rgb(58,58,58);font-family:Arial,serif;font-size:16px;line-height:26px;">I took this section from the wikipedia article, and while they're genetically near identical (Which, even notes there are small differences in their dna as I said), they're not the same. The real science around it is the same egg splits into two embyros, so they contain the same genes, but small genetic differences happen such as the finger prints and such.

<p style="margin-top:0.5em;margin-bottom:0.5em;color:rgb(37,37,37);font-family:sans-serif;"><span style="color:rgb(58,58,58);font-family:Arial,serif;font-size:16px;line-height:26px;">TLDR: Identical twins aren't clones, just near identical. Mabel and Dipper could be identical but admittedly probably aren't because it's unusual for identical twins to not be the same sex, but Ford and Lee probably are identical as they could easily have the same genetic makeup, just different phenotypes i.e. personalities and such.

<p style="margin-top:0.5em;margin-bottom:0.5em;color:rgb(37,37,37);font-family:sans-serif;"><span style="color:rgb(58,58,58);font-family:Arial,serif;font-size:16px;line-height:26px;">Also excuse me if my science is a little bit off, been a while since I've needed to even think about genetics, but yes I do remember that while identical twins are near identical in dna, they do have small differences.