TPI activity compared by race and genotype. When TPI activity of African Americans (AA) with no variant substitutions is compared with that of AA with the (−5 −8)/normal genotype (distributions shown in darker lines for emphasis), a clear-cut reduction in enzyme activity is apparent in the variant group. However, it is also noteworthy that decreased enzyme activity is not restricted to the range for previously established heterozygotes, which approximates one half of normal mean activity (AA normal/normal: mean = 2002.7 IU/g hgb, standard deviation (SD) = 454.4; AA [−5 −8]/normal: mean = 1700.2, SD = 393.9). These differences are highly significant (P = 8.6 × 10⁻⁸,t-test). TPI activity in the total group of AA is less than in whites (AA all genotypes: mean = 1912.9 IU/g hgb, SD = 442.6; white normal/normal: mean = 2041.9, SD = 366.3). This difference is significant (P = .008, t-test). However, when TPI enzyme activity of AA with no variant substitutions is compared with the white group, the differences between the two groups diminish and are probably of no significance (AA normal/normal: mean = 2002.7 IU/g hgb, SD = 454.4; white normal/normal: mean = 2041.9, SD = 366.3; P = .444, t-test). When enzyme activity of AA with no variant substitutions is compared with AA with the −5/normal genotype, the variant genotype is associated with a modest reduction in enzyme activity (AA normal/normal: mean = 2002.7 IU/g hgb, SD = 454.4; AA −5/normal: mean = 1867.7, SD = 376.2). These differences are significant (P = .008,t-test).