Blood is thicker than water

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Abstract
By contrast, in the absence of nonpaternity, case-unaffected sibling pairs are matched for ethnicity. Therefore, statistical methods have been developed to compare the frequency of risk factors in discordant sibling pairs (e.g., sibling transmission disequilibrium test).2 Although family-based association studies have been employed primarily for the study of genetic factors, the same design can be employed for the study of environmental factors.3