If monozygotic twins resemble each other on some dimension more than dizygotic twins do, researchers generally conclude there is a genetic basis for that dimension.

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Monozygotic twins come from a single fertilized egg that splits into two, whereas dizygotic twins come from two separate eggs fertilized by two separate sperm. Since monozygotic twins share 100% of their genes, any resemblances between them are considered to have a genetic basis, whereas any resemblance between dizygotic twins may be due to shared environmental factors as well as genetic factors.

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