Different phenotypes of heterozygotes for red blood cell enzymopathies. In a heterozygote for PK deficiency, encoded by an autosomal gene (Table 1), the level of enzyme is about one-half of normal in all red blood cells. Since this level of enzyme is sufficient, there are no clinical consequences—ie, PK deficiency is recessive. In a heterozygote for deficiency of G6PD, encoded by an X-linked gene, the situation is quite different: X chromosome inactivation generates red blood cell mosaicism, whereby some red blood cells are entirely normal, and others are G6PD deficient. Therefore, G6PD deficiency is expressed in heterozygotes: it is not recessive.