Isoenzyme / Polymorphism
Definition
Different molecular forms of the same enzyme separable by electrophoresis. Forensic polymorphic markers have several common alleles in the population, so a phenotype narrows down a donor.
- Detection method
- Electrophoretic separation
- Genetic basis
- Multiple common alleles per locus
- Forensic role
- Narrows possible donors by phenotype
- Predates
- Modern STR DNA profiling
Common questions
What does a polymorphic enzyme marker actually exclude?+
A phenotype result excludes any individual whose known enzyme type does not match the stain, and it statistically narrows the pool of possible donors using population frequency data for that marker, but it does not individualise a sample the way a full STR profile does.
Why combine several isoenzyme markers rather than rely on one?+
Any single marker has only a few common phenotypes, so the exclusionary power is limited. Testing a panel of independent polymorphic enzyme systems and multiplying their individual frequencies gives a combined discrimination power closer to, though still well below, DNA profiling.
Related terms
- Acid Phosphatase (AP)
- An enzyme present at high concentrations in seminal plasma. The Brentamine Fast Violet B (or SERI AP) spray test detects acid phosphatase...
- Alpha-Amylase
- Salivary enzyme. Detected by starch-iodine (clear zone in blue background) or the Phadebas reagent (blue colour from dye-linked starch). Concentration in saliva...
- Barberio Test
- Presumptive semen test. Saturated picric acid reacts with spermine to form yellow rhombic spermine picrate crystals.
- Florence Test
- Presumptive semen test. Iodine plus potassium iodide reacts with choline in semen to form dark-brown rhombic / needle-shaped choline iodide crystals.
- Hp, Gc, Tf
- The classical serum-protein polymorphism panel: haptoglobin (binds free haemoglobin; Hp1-1, Hp2-1, Hp2-2 phenotypes), group-specific component (vitamin D binding protein, Gc1F / Gc1S...
- P30 (PSA)
- Prostate-specific antigen, a glycoprotein secreted by the male prostate gland and present in semen at high concentrations. ELISA or lateral-flow immunochromatographic tests...
- PGM, EsD, AK, GLO
- The classical RBC isoenzyme panel: phosphoglucomutase, esterase D, adenylate kinase, glyoxalase I. All polymorphic, all typed by starch-gel or isoelectric-focusing electrophoresis of...
- Secretor
- An individual whose ABO blood group antigens are expressed on red blood cells and also released into body fluids such as saliva,...
- Spermatozoa
- Mature male gametes. Microscopic identification of intact spermatozoa with the characteristic head, midpiece and tail is the gold-standard confirmatory for semen in...