Hemoglobin variants are a part of the normal embryonic and fetal development. They may also be pathologic mutant forms of hemoglobin in a population, caused by variations in genetics. Some well-known hemoglobin variants, such as sickle-cell anemia, are responsible for diseases and are considered hemoglobinopathies. Other variants cause no detectable pathology, and are thus considered non-pathological variants.
In embryos:
* Gower 1 (ζε).
* Gower 2 (αε) ().
* Hemoglobin Portland I (ζγ).
* Hemoglobin Portland II (ζβ).
In fetuses:
* Hemoglobin F (αγ) ().
In neonates (newborns inmmediately after birth):
* Hemoglobin A (adult hemoglobin) (αβ) () – The most common with a normal amount over 95%
* Hemoglobin A (αδ) – δ chain synthesis begins late in the third trimester and, in adults, it has a normal range of 1.5–3.5%
* Hemoglobin F (fetal hemoglobin) (αγ) – In adults Hemoglobin F is restricted to a limited population of red cells called F-cells. However, the level of Hb F can be elevated in persons with sickle-cell disease and beta-thalassemia.
Abnormal forms that occur in diseases:
* Hemoglobin D – (αβ) – A variant form of hemoglobin.
* Hemoglobin H (β) – A variant form of hemoglobin, formed by a tetramer of β chains, which may be present in variants of α thalassemia.
* Hemoglobin Barts (γ) – A variant form of hemoglobin, formed by a tetramer of γ chains, which may be present in variants of α thalassemia.
* Hemoglobin S (αβ) – A variant form of hemoglobin found in people with sickle cell disease. There is a variation in the β-chain gene, causing a change in the properties of hemoglobin, which results in sickling of red blood cells.
* Hemoglobin C (αβ) – Another variant due to a variation in the β-chain gene. This variant causes a mild chronic hemolytic anemia.
* Hemoglobin E (αβ) – Another variant due to a variation in the β-chain gene. This variant causes a mild chronic hemolytic anemia.
* Hemoglobin AS – A heterozygous form causing sickle cell trait with one adult gene and one sickle cell disease gene
* Hemoglobin SC disease – A compound heterozygous form with one sickle gene and another encoding Hemoglobin C.
* Hemoglobin Hopkins-2 – A variant form of hemoglobin that is sometimes viewed in combination with Hemoglobin S to produce sickle cell disease.