Types of Sickle Cell Disease: From Trait to Anemia

Sickle cell disease is not a single condition – it is a family of inherited blood disorders, each shaped by the specific combination of genes a person inherits from their parents. Understanding how these gene combinations work is the first step to making sense of why some people with sickle cell disease have severe daily symptoms while others live relatively normal lives. The difference often comes down to which genes were passed on and how they interact within the body’s red blood cells.

How genes shape sickle cell disease
Sickle cell trait (HbAS): being a carrier
When sickle cell trait can cause problems
Sickle cell anemia (HbSS): the most severe form
HbSC disease: a milder but not mild form
Specific complications of HbSC
HbS beta-thalassemia: when two blood disorders combine
Rare variants: HbSD, HbSE, and HbSO
Why the type matters

How genes shape sickle cell disease

Every person inherits two copies of the gene responsible for producing beta-globin, a protein that forms part of hemoglobin – the molecule in red blood cells that carries oxygen. One copy comes from each biological parent. A specific mutation in the beta-globin gene (HBB) produces an abnormal protein known as hemoglobin S (HbS), which causes red blood cells to become rigid and crescent-shaped rather than smooth and flexible.

In genetics, a dominant gene is one that is expressed when even one copy is present, while a recessive gene requires two copies – one from each parent – before its effects become visible. Sickle cell disease follows an autosomal recessive inheritance pattern, meaning a person must inherit two altered gene copies to develop the full disease. When only one copy of the HbS gene is inherited alongside a normal gene, the result is quite different – the person becomes a carrier, not a patient.

Sickle cell trait (HbAS): being a carrier

Sickle cell trait occurs when a person inherits one normal hemoglobin gene (A) from one parent and one sickle hemoglobin gene (S) from the other, resulting in the genotype HbAS. This is not a disease. People with sickle cell trait are carriers – their blood contains both normal hemoglobin (HbA) and a smaller proportion of sickle hemoglobin (HbS), but the normal gene produces enough healthy hemoglobin to prevent disease symptoms under most conditions.

Individuals with sickle cell trait carry only one defective gene and typically live normal lives. In fact, approximately 300 million people worldwide are estimated to have sickle cell trait, with one-third of this population living in sub-Saharan Africa – regions where the trait historically offered some protection against severe malaria.

The critical point for carriers is not their own health – it is the genes they can pass on. If two people with sickle cell trait (HbAS) have children together, there is a one in four chance with each pregnancy that the child will inherit sickle cell anemia (HbSS), a one in two chance the child will be a carrier like their parents, and a one in four chance the child will have entirely normal hemoglobin. Carriers may be unaware of their status since they experience no symptoms, which is why genetic screening and counseling are so important.

When sickle cell trait can cause problems

While HbAS is largely a benign carrier state, rare complications can occur under extreme conditions such as severe dehydration and high-intensity physical activity, and have occasionally led to serious health issues. These situations increase the likelihood of red blood cells sickling, even when only one copy of the HbS gene is present. This is why carriers involved in strenuous military training or elite athletics are advised to take specific precautions around hydration and exercise intensity.

Sickle cell anemia (HbSS): the most severe form

People with sickle cell anemia inherit a hemoglobin S gene from each parent, giving them the HbSS genotype. Because both copies of the beta-globin gene are affected, virtually all of their hemoglobin is the abnormal sickle type. This is the most common and most severe form of sickle cell disease.

More than 60% of people with sickle cell disease have this HbSS type. With almost no normal hemoglobin available, red blood cells sickle extensively, blocking small blood vessels and causing repeated episodes of intense pain known as vaso-occlusive crises. Over time, these blockages damage organs including the spleen, kidneys, lungs, and brain. Chronic anemia – a persistent shortage of healthy red blood cells – is also a hallmark of the condition, since sickled cells break down far more rapidly than normal cells.

Despite being the most severe variant, HbSS is also the most studied. Treatment options including blood transfusions, hydroxyurea, L-glutamine, and newer gene therapies have improved outcomes considerably, though managing the condition still demands intensive, lifelong medical care.

HbSC disease: a milder but not mild form

In HbSC disease, one copy of the HBB gene makes hemoglobin S while the other produces a different abnormal type, hemoglobin C. HbC is caused by a different mutation in the same gene – it is not the normal hemoglobin A, but it is not as damaging as HbS on its own. When HbS and HbC are inherited together, however, the result is a form of sickle cell disease that is generally milder than HbSS but still carries significant risks.

HbSC is the second most common form of sickle cell disease in the United States, accounting for approximately 25% of patients. For a long time, clinicians regarded it as a “mild” variant, particularly in childhood. However, recent research published in the British Journal of Haematology found that HbSC disease is more clinically severe in adults than previously recognized, with complications that can be just as life-limiting as those seen in HbSS.

Specific complications of HbSC

Two complications are especially associated with HbSC. The first is proliferative sickle cell retinopathy – abnormal blood vessel growth in the retina that can lead to vision loss. This retinopathy occurs in 30-70% of people with HbSC disease, compared to only 3% in HbSS, making it one of the most distinctive features of this genotype. The second is osteonecrosis – a painful condition where reduced blood flow to bones in joints causes bone tissue to break down faster than the body can rebuild it. These specific complications stem from the increased thickness and viscosity of blood in HbSC, which creates particular problems in the small vessels of the eye and bone joints.

HbS beta-thalassemia: when two blood disorders combine

Beta-thalassemia is a separate inherited blood disorder that reduces or eliminates normal production of beta-globin. When a person inherits one HbS gene from one parent and one beta-thalassemia gene from the other, the result is HbS beta-thalassemia – a form of sickle cell disease that combines characteristics of both conditions.

There are two subtypes: HbS beta-zero (HbSβ0) thalassemia, in which no normal hemoglobin is produced, and HbS beta-plus (HbSβ+) thalassemia, in which a small amount of normal hemoglobin is still made. This distinction matters greatly for disease severity. People with HbSβ0-thalassemia tend to experience complications similar in severity to those with HbSS, while HbSβ+-thalassemia usually produces milder symptoms because the partial normal hemoglobin production offers some protection against extensive sickling.

Severe and moderate forms of HbS beta-thalassemia are most common in the eastern Mediterranean region and parts of India, while milder forms are more prevalent in populations of African ancestry. Like all forms of sickle cell disease, diagnosis is typically confirmed through newborn blood screening, which can identify the hemoglobin genotype shortly after birth.

Rare variants: HbSD, HbSE, and HbSO

Beyond the three main types, there are several rarer forms of sickle cell disease. These occur when a person inherits one HbS gene alongside a gene for another abnormal hemoglobin type – such as hemoglobin D, E, or O – resulting in genotypes like HbSD, HbSE, or HbSO. The severity of these rarer types varies depending on how the particular hemoglobin variant interacts with HbS, and they are less well studied due to their lower prevalence.

Why the type matters

The genotype a person inherits directly shapes their clinical experience – how severe their anemia is, what complications they are at risk for, and what their long-term health trajectory looks like. While all forms of sickle cell disease can cause anemia, only HbSS and HbSβ0-thalassemia are technically classified as “sickle cell anemia” and represent the most severe end of the spectrum. HbSC and HbSβ+-thalassemia sit in the middle – still serious conditions requiring regular medical monitoring, but with somewhat more moderate baseline anemia.

For educators, healthcare workers, families, and anyone supporting people with sickle cell disease, understanding these distinctions is not just academic. It informs how people are screened, how carriers make family planning decisions, what complications to watch for, and what kind of care different individuals will need over a lifetime.

What do you think? If two parents are both carriers of sickle cell trait but show no symptoms, how might understanding their genetic status change the decisions they make before or during pregnancy? And given that HbSC was long considered a “mild” form of sickle cell disease – only for recent research to challenge that view – what does this suggest about the importance of continued study into less common variants of inherited conditions?

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Table of Contents

How genes shape sickle cell disease 🔗

Sickle cell trait (HbAS): being a carrier 🔗

When sickle cell trait can cause problems 🔗

Sickle cell anemia (HbSS): the most severe form 🔗

HbSC disease: a milder but not mild form 🔗

Specific complications of HbSC 🔗

HbS beta-thalassemia: when two blood disorders combine 🔗

Rare variants: HbSD, HbSE, and HbSO 🔗

Why the type matters 🔗