Sickle-cell trait
| Sickle cell trait | |
|---|---|
 |
|
| Sickle cells in human blood: both normal red blood cells and sickle-shaped cells are present | |
| Classification and external resources | |
| Specialty | hematology |
| ICD-10 | D57.3 |
| ICD-9-CM | 282.5 |
| OMIM | 603903 |
| MedlinePlus | 000527 |
| eMedicine | topic list |
| MeSH | D012805 |
Sickle cell trait describes a condition in which a person has one abnormal allele of the hemoglobin beta gene (is heterozygous), but does not display the severe symptoms of sickle-cell disease that occur in a person who has two copies of that allele (is homozygous). Those who are heterozygous for the sickle cell allele produce both normal and abnormal hemoglobin (the two alleles are codominant with respect to the actual concentration of hemoglobin in the circulating cells).
Sickle cell disease is a blood disorder in which there is a single amino acid substitution in the hemoglobin protein of the red blood cells, which causes these cells to assume a sickle shape, especially when under low oxygen tension. Sickling and sickle cell disease also confer some resistance to malaria parasitization of red blood cells, so that individuals with sickle-cell trait (heterozygotes) have a selective advantage in environments where malaria is present.
Normally, a person inherits two copies of the gene that produces beta-globin, a protein needed to produce normal hemoglobin (hemoglobin A, genotype AA). A person with sickle cell trait inherits one normal allele and one abnormal allele encoding hemoglobin S (hemoglobin genotype AS).
The sickle cell trait can be used to demonstrate the concepts of co-dominance and incomplete dominance. An individual with the sickle cell trait shows incomplete dominance when the shape of the red blood cell is considered. This is because the sickling happens only at low oxygen concentrations. With regards to the actual concentration of hemoglobin in the circulating cells, the alleles demonstrate co-dominance as both 'normal' and mutant forms co-exist in the blood stream. It is interesting to note that unlike the sickle-cell trait, sickle-cell disease is passed on in a recessive manner.
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