The Mode of Inheritance of PTA Deficiency: Evidence for the Existence of Major PTA Deficiency and Minor PTA Deficiency

Plasma thromboplastin antecedent (PTA) activity was measured with a quantitative assay in the available members of the families of eight propositi with severe PTA deficiency. PTA deficiency was found to exist in two forms: major PTA deficiency, characterized by PTA levels of up to 20 per cent of our standard reference plasma and by the potential for serious surgical bleeding, and minor PTA deficiency, characterized by PTA levels between 30 and 65 per cent of our standard reference plasma and by the absence of significant surgical bleeding. Minor PTA deficiency was found in parents and children of subjects with major PTA deficiency.

It would appear that the gene for PTA deficiency is an incompletely recessive or "intermediate" gene which produces major PTA deficiency in the homozygote and minor PTA deficiency in the heterozygote.