Two French Canadian families among whom the Tay-Sachs allele is segregating are analyzed to investigate whether there is residual phenotypic familial covariation over and above that from major gene segregation. The traits analyzed are functions of hexosaminidase A (HEXA) and hexosaminidase B (HEXB) levels, and a combination of segregation and discriminant analysis is used to find a function of HEXA and HEXB levels to discriminate between carriers and noncarriers in this French Canadian population. We found that residual familial covariation is significant for the ratio HEXA/(HEXA + HEXB), the statistic generally used to determine if an individual is likely to be a carrier of the Tay-Sachs allele in Ashkenazi Jewish populations. This suggests that the ratio may not be appropriate for the French Canadian group and that the mutation may be different from that in Ashkenazi Jews. A linear function is calculated that gives improved discrimination between carriers and noncarriers for the two French Canadian pedigrees studied.