Protein in the blood of many animals (in vertebrates it is in red blood cells) that transports oxygen from the lungs to the tissues. It is bright red when combined with oxygen and purple-blue in the deoxygenated state. Each molecule is made up of a globin (a type of protein) and four heme groups. Heme, a complex heterocyclic compound, is an organic molecule derived from porphyrin with an iron atom at the centre. Variant hemoglobins (see sickle-cell anemia; hemoglobinopathy) can be used to trace past human migrations and to study genetic relationships among populations.
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