If identical twins share a single placenta rather than having separate placentas, their chances of being similarly affected by are dramatically increased.

Schizophrenia.

Explanation:

In this case, the genetical charge of schizophrenia is very high. Schizophrenia affects both members of identical twins in between 40 and 60 percent of cases. The heritability of schizophrenia in twins is nearly 80 percent as well. That's why, if identical twins share a single placenta, the chances are dramatically increased due to the heritability factors that "heightened" in the same placenta.

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