Understanding Your Results
Do you carry an abnormal hemoglobin gene?
Find out if you are at risk - get tested
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Possible Outcomes
We each inherit two copies of the HBB gene – one from each parent. This DNA analysis distinguishes between four different alleles of HBB – one normal allele (HbA) and three abnormal alleles (HBS, HbC and HbE). There are several possible combinations of these four alleles.
- Normal individual – homozygous HbA/HbA
A normal result means that the person carries two normal copies of HBB. This is not associated with sickle cell disease. There is a 100% chance that this person will pass a normal copy of the HBB gene to the next generation.
- Sickle cell anemia or hemoglobin HbSS disease – homozygous HbS/HbS (20T/20T)
This person has two defective copies of HBB, both with the c.20A>T mutation that produces HbS. They have sickle cell anemia (hemoglobin SS disease) and will have sickle shaped red blood cells and the severe symptoms associated with the disease. There is a 100% chance that this person will pass on a defective copy of the HbS allele to the next generation.
- Hemoglobin SC disease – compound heterozygous HbS/HbC (20A/20T, 19G/19A)
This person has two defective copies of HBB, one with the c.20A>T mutation that produces HbS and the other with the c.19G>A mutation that produces HbC. They have hemoglobin SC disease and will experience the symptoms associated with the disease (generally milder symptoms than HbSS disease). There is a 100% chance of passing on a defective copy of HBB (with either the c.20A>T mutation or the c.19G>A mutation) to the next generation.
- Hemoglobin SE disease – compound heterozygous HbS/HbE (20A/20T, 79G/79A)
This person has two defective copies of HBB, one with the c.20A>T mutation that produces HbS and the other with the c.79G>A mutation that produces HbE. They have hemoglobin SE disease and will experience the symptoms associated with the disease (generally milder symptoms than HbSS disease). There is a 100% chance of passing on a defective copy of HBB (with either the c.20A>T mutation or the c.79G>A mutation) to the next generation.
- Sickle cell trait – heterozygous HbA/HbS (20A/20T)
This person has the sickle cell trait, which means they have inherited one normal copy of HBB and one defective copy that carries the c.20A>T mutation that will make HbS. This person has some sickled cells but will still produce sufficient normal hemoglobin and is unlikely to experience symptoms of the disease. There is a 50% chance that this person will pass on the defective copy of HBB with the c.20A>T mutation to the next generation.
- Hemoglobin C disease – homozygous HbC/HbC (19A/19A)
This person has two defective copies of HBB, both with the c.19G>A mutation that produces HbC. This disease is associated with only mild anemia or no symptoms are apparent. There is a 100% chance that this person will pass on a defective copy of the HbC allele to the next generation.
- Hemoglobin E disease – homozygous HbE/HbE (79A/79A)
This person has two defective copies of HBB, both with the c.79G>A mutation that produces HbE. This disease is associated with only mild anemia or no symptoms are apparent. There is a 100% chance that this person will pass on a defective copy of the HbE allele to the next generation.
- Hemoglobin CE disease – compound heterozygous HbC/HbE (19G/19A, 79G/79A)
This person has two defective copies of HBB, one with the c.19G>A mutation that produces HbC and the other with the c.79G>A mutation that produces HbE. They have hemoglobin SC disease and will experience mild anemia symptoms. There is a 100% chance of passing on a defective copy of HBB (with either the c.19G>A mutation or the c.79G>A mutation) to the next generation.
- Hemoglobin C trait – heterozygous HbA/HbC (19G/19A)
This person has the hemoglobin C trait, which means they have inherited one normal copy of HBB and one defective copy that carries the c.19G>A mutation that will make HbC. This person does not have any disease symptoms. There is a 50% chance that this person will pass on the defective copy of HBB with the c.19G>A mutation to the next generation.
- Hemoglobin E trait – heterozygous HbA/HbE (79G/79A)
This person has the hemoglobin E trait, which means they have inherited one normal copy of HBB and one defective copy that carries the c.79G>A mutation that will make HbE. This person does not have any disease symptoms. There is a 50% chance that this person will pass on the defective copy of HBB with the c.79G>A mutation to the next generation.
- Other possible HbS/HbC/HbE combinations
For each of the compound heterozygous diseases above (HbSC, HbSE and HbCE), it is assumed that each mutation occurs on a different copy of the HBB gene (i.e. in trans), so that both copies of the HBB gene are affected. Confirmation of this trans inheritance can only be achieved by DNA analysis of the parent’s of the affected individual. If both mutations have occurred in cis (on the same copy of the HBB gene), the individual will still have one normal HbA allele and is likely to be unaffected.
There are several further combinations that can occur when more than one mutation occurs in the same copy of the HBB gene (in cis). For example, a person could have one copy with both the HbS mutation (c.20A>T) and the HbE mutation (c.79G>A), and the second copy could also have HbS mutation (c.20A>T). This person would have sickle cell disease (due to two copies of the HbS c.20A>T mutation) and may suffer from more severe symptoms due to presence of the third mutation (c.79G>A).
Other Hemoglobin Diseases
Other hemoglobin disorders occur when a person inherits other mutations in the α-globin (HBA) and/or β-globin (HBB) genes. These include alpha thalassemias, beta thalassemias and sickle/thalassemia combinations and the disease symptoms depend on the number and severity of the inherited mutations. Mutations other than c.20A>T, c.19G>A and c.79G>A (all in the HBB gene) are not detected in this analysis.