Red cell glucose‐6‐phosphate dehydrogenase activity in individuals with abnormal numbers of X‐chromosomes

SUMMARY Red cell glucose‐6‐phosphate dehydrogenase (G‐6‐PD) activity was measured in fourteen XXX, eleven XX Y, three XO individuals, four individuals who were 17 but had testicular feminization, and in appropriate male and female controls. No evidence for any quantitative relationship between the number of X‐chromosomes present and the level of red‐cell G‐6‐PD activity was observed. Since G‐6‐PD is known to be determined by at least one gene locus in the X‐chromosome, these results in agreement with others imply complete dosage compensation. In thirteen patients with thyrotoxicosis a significant elevation of the mean level of red cell G‐6‐PD was demonstrated, indicating that such increases could be detected under the general conditions of the investigation. The results are discussed in terms of various hypotheses accounting for dosage compensation.