Some of the proteins circulating in serum display detectable polymorphisms, with alleles that have sufficient frequency differences to be of value in blood typing.  Transferrin (Tf) and Group Specific Component (Gc) were two that offered considerable promise and were becoming routinely used just before the advent of DNA typing.  However, haptoglobin (Hp) was the most widely used of the polymorphic serum proteins in forensic biology.

Haptoglobin is a hemoglobin-binding protein found in the α-globulin fraction of serum.  There are two alleles, designated Hp 1 and Hp 2, with several rare variants at each allele.  The alleles are separated by electrophoresis on a gradient polyacrylamide gel (that is, one in which the concentration of polyacrylamide varies from 5% at the top to 30% at the bottom, so giving enhanced separation by molecular sieving).

Haptoglobin 1 is a monomer consisting of two pairs of peptide chains (α and β) joined by disulfide bridges.  Electrophoresis of serum from someone who is homozygous for Hp 1 shows only one band.  In contrast, samples from someone who is homozygous for Hp 2 display multiple bands on electrophoresis.  Curiously, electrophoresis of a sample from a heterozygous Hp 2-1 shows a band matching the Hp 1 band along with multiple other bands, but these do not align exactly with those from a haptoglobin 2 homozygous person.  The Hp 2 proteins are similar to Hp 1 in that they are composed of α and β peptide chains cross-linked by disulfide bridges.  However, the α peptides (α²) are not the same as those in Hp 1.  Furthermore, the proteins are found as polymers of the structure α²_nβ_n where n is between 3 and 8.  In heterozygotes, some of the polymers incorporate α¹ chains as well as α² ones.^03, 04,

Haptoglobin is a reasonably good system for use in forensic serology. It is stable in stains and the assay is quite sensitive, using one of the hemoglobin screening procedures, such as leucomalachite green, to visualize the bands by reacting with the bound hemoglobin.

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