Although monozygotic twins are genetically almost identical, a 2012 study of 92 pairs of monozygotic twins found that monozygotic twins acquire hundreds of genetic differences early in … This occurs when two sperm cells fertilize one egg which then later splits. The only way to be sure about zygosity is to do a DNA test. Twins that are born as the same genetic sex are most likely to be monozygotic. There is an extremely rare twin type that’s referred to as semi-identical twins or sesquizygotic twins. Fraternal twins may be of the same or different sexes. Semi-identical twins share anywhere from 50 to 100 percent of their DNA. Very rarely, the zygote splits around day 13-15, making it impossible for the twins to separate fully. Identical twins share all of their genes and are always the same sex. Identical twins may not be carbon copies at the DNA level after all. This causes the babies to be born conjoined. 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