Gene dosage effect for the enzyme peptidase A was studied in the red cells of subjects trisomic (seven cases) or monosomic (five cases) for the segment of chromosome 18 carrying the gene. The individual levels of enzyme activity in both groups were different from those of the controls, but with a wide overlap. The use of the ratio of the activity of each subject to the midparent activity eliminated the overlapping. The mean ratio was 0.94 for the controls, 2.36 for the trisomics, and 0.41 for the monosomics. The trisomic ratio is higher than expected on the assumption of a linear effect. Correlation with the cytogenetic data in four cases of ring 18 and one of 18q-firmly places the gene for peptidase A on band 18q23.
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