RFLP of the X chromosome-linked glucose-6-phosphate dehydrogenase locus in blacks.

The X chromosome-linked glucose-6-phosphate dehydrogenase (G6PD) A(+) variant is found in approximately 20% of blacks. Examination of the structure of the G6PD A(+) gene revealed that AT----GC transition occurred in the variant gene, resulting in the amino acid substitution Asn----Asp at the one hundred forty-second position from the NH2-terminal of the enzyme (Takizawa and Yoshida 1987). The nucleotide change created an additional FokI cleavage site in the variant A(+) gene; thus, the FokI fragment type of the variant A(+) DNA differs from that of the normal B(+) DNA. PvuII fragment type is also found to be polymorphic in blacks, but not in Caucasians. The majority of blacks, as well all nonblacks, have a major hybridization-positive fragment of approximately 4.0 kbp (PvuII type 1), while approximately 20% of blacks have a major fragment of approximately 1.5 kbp (PvuII type 2). The G6PD gene with PvuII type 2 contains an additional PvuII cleavage site approximately 0.7 kbp downstream from the mutation site of the G6PD A(+). Approximately 40% of the G6PD A(+) genes have PvuII type 2, while only approximately 10% of the G6PD B(+) genes are associated with PvuII type 2. The data indicate a statistically significant (X2 = 6.85, P less than .020) linkage disequilibrium between the G6PD types and the PvuII types at the G6PD locus.