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twin |
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twinOne of two young produced from a single pregnancy. Human twins may be genetically identical (monozygotic), having been formed from a single fertilized egg that splits into two cells, both of which became implanted. Nonidentical (fraternal or dizygotic) twins are formed when two eggs are fertilized at the same time.
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Interestingly, in patients younger than 50, concordance was dramatically different between monozygotic twins (where four of four twin pairs were concordant) and dizygotic twins (where only 2 of 12 twin pairs were concordant). Twin studies show a higher concordance among monozygotic twins (85%) than among dizygotic twins (21%). Like dizygotic twins, ordinary siblings also share approximately half of their genes; genetically, dizygotic twins are ordinary siblings. |
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