In short: Your blood group (A, B, AB or O, RhD positive or negative) is about transfusion compatibility, while your haemoglobin genotype (such as AA, AS or SS) shows whether you have or carry sickle cell disease. They are inherited separately and tested differently. Knowing both helps with transfusions, pregnancy and family planning.
Introduction
When people talk about health and genetics, two terms often come up: blood group and haemoglobin genotype. Both are fundamental parts of our biological make-up, but they serve different purposes and have different implications for health. Here we explain what each term means, how they differ and why understanding the difference matters.
What Is a Blood Group?
Definition: Blood group, or blood type, is the classification of blood based on the presence or absence of certain antigens on the surface of red blood cells. The most widely used systems are the ABO system and the Rh system.
The ABO system:
- Group A has A antigens.
- Group B has B antigens.
- Group AB has both A and B antigens.
- Group O has neither A nor B antigens.
The Rh system:
- Rh-positive (Rh+) people have the RhD antigen.
- Rh-negative (Rh-) people do not have the RhD antigen.
Why it matters:
- It is critical for blood transfusions, to avoid transfusion reactions.
- It influences organ transplant compatibility.
- It can affect pregnancy if the mother is Rh-negative and the baby is Rh-positive.
What Is Haemoglobin Genotype?
Definition: Haemoglobin genotype describes the genetic make-up that determines which type of haemoglobin you produce. Haemoglobin carries oxygen in the blood. The most commonly discussed genotypes relate to normal adult haemoglobin (HbA) and variants such as HbS (sickle haemoglobin) and HbC.
Common genotypes:
- AA: Normal haemoglobin; no sickle cell trait or disease.
- AS: Sickle cell trait (carrier); one normal gene and one sickle gene.
- SS: Sickle cell anaemia; both genes code for sickle haemoglobin.
- Sβ+ or Sβ0: Sickle beta-thalassaemia; one sickle gene and one beta-thalassaemia gene.
- SC, SD, SE, SO: Sickle cell disease caused by a sickle gene plus another abnormal haemoglobin (HbC, HbD, HbE or HbO).
- AC: HbC trait (carrier), usually without symptoms.
- CC: Homozygous HbC disease, usually mild.
Why it matters:
- It determines the risk of sickle cell disease, in which red blood cells can become sickle-shaped and cause serious health problems.
- It informs carrier status and family planning. Sickle cell trait also offers some protection against severe malaria, which is why it is common in regions where malaria is or was widespread.
Key Differences
- Function: Blood group determines compatibility for transfusions and other procedures. Haemoglobin genotype defines the type of haemoglobin you make, which affects oxygen transport and related conditions.
- Genetics: Blood group is determined by the ABO gene and, for the Rh factor, the RHD and RHCE genes. Haemoglobin genotype is inherited from both parents and, for sickle cell and related variants, involves the beta-globin (HBB) gene.
- Health implications: Blood group mainly matters for transfusions, transplants and pregnancy, with only modest links to some diseases. Haemoglobin genotype directly affects health, especially sickle cell conditions, and has implications for carrier status and disease management.
- Testing: Blood group is found by blood typing, which uses specific antibodies to detect antigens on red blood cells. Haemoglobin genotype is found with tests such as high-performance liquid chromatography (HPLC), haemoglobin electrophoresis or genetic (DNA) testing.
Why You Should Know the Difference
Understanding these differences isn't just for medical professionals. It is useful for everyone:
- Blood donation and transfusion: Knowing your blood group can make you a potential lifesaver through blood donation, and it is crucial for safe transfusions.
- Family planning: Knowing your haemoglobin genotype, and your partner's, can inform family planning decisions, especially in communities where sickle cell disease is common.
- Health management: If you carry the sickle cell trait or have a sickle cell condition, knowing your genotype helps you and your healthcare team manage your health.
Genotype Compatibility Guide
Your haemoglobin genotype shows whether you could pass on sickle cell disease. Here is what the common genotypes mean for family planning:
- AA: no sickle gene. An AA person cannot have a child with sickle cell disease, whatever their partner's genotype, though children may be carriers.
- AS (sickle cell trait): a healthy carrier. Two AS parents have a 1 in 4 chance in each pregnancy of a child with sickle cell disease (SS).
- AC (HbC trait): a healthy carrier. With an AS partner, each pregnancy has a 1 in 4 chance of SC disease.
- SS: sickle cell anaemia. Every child will inherit at least one S gene.
- SC: a form of sickle cell disease. Every child will inherit either S or C.
Can AS Marry AS?
Yes. Two carriers (AS and AS) can marry and have children. The important thing is to know the risk: in each pregnancy there is a 25% (1 in 4) chance the child will be SS (sickle cell disease), a 50% chance of AS (carrier) and a 25% chance of AA. The chances are the same every pregnancy. Couples who are both carriers should speak to their GP or a genetic counsellor about their options, which can include prenatal testing during pregnancy and, in some cases, IVF with preimplantation genetic testing (PGT-M).
Genotype Inheritance Chances
Chances for each pregnancy:
- AA × AA: all children AA
- AA × AS: 50% AA, 50% AS
- AA × SS: all children AS (carriers)
- AS × AS: 25% AA, 50% AS, 25% SS
- AS × SS: 50% AS, 50% SS
- AS × AC: 25% AA, 25% AS, 25% AC, 25% SC
- AC × AC: 25% AA, 50% AC, 25% CC
- SS × SS: all children SS
Find out more on the NHS sickle cell disease page and in our guide to premarital testing.
Conclusion
Blood group and haemoglobin genotype both relate to your blood, but they are distinct. Blood group concerns how your blood interacts with other people's blood, which is crucial for medical procedures. Haemoglobin genotype concerns the haemoglobin that carries oxygen around your body and directly influences your health. Both are inherited, but their implications differ greatly. Knowing both can support better health decisions, especially around transfusions, genetic counselling and managing inherited conditions.
Frequently asked questions
Can AS marry AS?
Yes. But in each pregnancy there is a 1 in 4 chance of a child with sickle cell disease (SS). Couples should seek genetic counselling to understand their options.
Is blood group related to genotype?
No. They are controlled by different genes. Any blood group can occur with any genotype.
Can AA marry SS?
Yes. All their children will be AS carriers, who are usually healthy, but none will have sickle cell disease.
Which genotypes are compatible?
Any couple where at least one partner is AA cannot have a child with sickle cell disease. Risk arises when both partners carry an abnormal gene (for example AS × AS, AS × AC, AS × SS).
How do I find out my genotype?
With a blood test such as haemoglobin HPLC or electrophoresis. In England, pregnant women and newborn babies are offered screening.
Book a test at E&J Bio
Walk-ins are welcome at our clinic at 305 Neasden Lane, London NW10 1QR. We are open 7 days a week, including Sundays, or you can book online at ejbio.co.uk/shop. Results for the single and couple Blood Group and Sickle Genotype tests are usually available the same day.
- Blood Group and Sickle Cell Genotype (AA, AS, SS) (results: same day)
- Blood Group and Sickle Genotype - Couple (results: same day)
- Blood Group, Sickle Cell Genotype Plus Thalassaemia Screen - Couple (results: next day)
- Haemoglobin Electrophoresis (results: same day)
This article is for general information only. Please discuss your results, and any symptoms, with a doctor or other qualified healthcare professional.
