Hemoglobin is the oxygen carrying substance in the blood. It is found in the red blood cells, and it is what gives blood its red color. It is a complex molecule consisting of 4 globin chains (polypeptide chains or proteins) with a heme group inserted into each globin "pocket". The heme group consists of a porphyrin ring and an iron (Fe) atom occupying the center of the ring. It is the iron atom that binds oxygen for transport through the blood. One hemoglobinn molecule carries 4 oxygen molecules.

The normal human hemoglobins found at different stages of development are encoded for by 6 different structural genes resulting in 6 globin (polypeptide) chains: alpha , beta , gamma , delta , epsilon , and zeta .

Normal adult hemoglobin (HbA) contains 2 globin chains and 2 globin chains.

Fetal hemoglobin (HbF) contains 2 and 2 globin chains.

The embryonic hemoglobins, Gower 1 (), Gower 2 () and Portland (), are synthesized in the very early embryo. At about 8 weeks of gestation the fetal liver takes over synthesizing predominantly HbF () and a small amount of HbA ().

An infant is born with approximately 70% HbF. The level normally falls to about 1% within the first year of life.

A normal adult will typically have:

HbA		>96%
HbF		<1%
HbA2		<3.5%

Elevated HbA₂ is characteristic of beta-thalassemia trait.