Mention has been made of the role of hemoglobin in screening and confirmatory tests for blood.  Hemoglobin is yet another protein formed from two pairs of polypeptide chains.  There are several variants of hemoglobin. All have the same structure for one of the pairs of polypeptide chains – designated as α.  The dominant form found in adult humans is termed Hemoglobin A (Hb A) and is composed of two α and two β chains.  About 2 to 3 per cent of human adult Hb consists of a variant called HbA₂ in which the β chains are replaced by two δ chains.  A more significant variant is HbF, which makes up about 70% of the hemoglobin in fetal blood.  HbF has a pair of γ chains instead of β.  HbF is rapidly replaced by HbA after birth, and only a trace remains by age 1 year.

Detection of HbF in a blood stain is an indicator of fetal blood.  The usual test is a combination of electrophoresis and the resistance of HbF to alkali denaturation.

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