The major problem with family incidence studies that ask whether diseases "run in families" is that even when they find a higher prevalence of a certain illness in families with one ill proband,

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The finding of a higher prevalence of a certain illness in families with one ill proband is consistent with both genetic and environmental causation. It does not allow for a determination if a single gene or multiple genes are involved, or if dominant or recessive genes are involved. Additionally, family incidence studies can include any type of family relationship, not just identical twins.

The Major Problem with Family Incidence Studies That Ask Whether