Pseudodeficiency alleles

A pseudodeficiency allele or pseudodeficiency mutation is a mutation that alters the protein product or changes the gene's expression, but without causing disease. For example, in the lysosomal storage diseases, patients with a pseudodeficiency allele show greatly reduced enzyme activity, yet they remain clinically healthy.
In medical genetics, a false positive result occurs in an enzyme assay test when test results are positive, but disease or morbidity is not present. One possible cause of false positive results is a pseudodeficiency allele. Disease may also be present, but at a subclinical level.
Implications.
A pseudodeficiency allele may indicate a deficiency of the enzyme assay method, or it may reflect incomplete understanding of the enzyme's activity.
Because of pseudodeficiency alleles, the results of enzyme assay testing in one population cannot be generalized to other populations. For example, while Tay–Sachs screening was able to nearly eliminate Tay–Sachs disease among Ashkenazi Jews, similar screening in the general population has proven less effective.
For some genetic diseases, especially those that have low penetrance or are easily treatable, carrier screening may be of questionable value when pseudodeficiency alleles are present at high frequency.