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If you and your partner are planning marriage or thinking about having children, you may have searched for genotype compatibility, AA and AS genotype crossing, or asked whether combinations such as AS + AS, AA + SS or AS + AC can affect your future children.
The important point to understand first is this: genotype compatibility is not really about deciding whether two adults are medically “compatible” or “incompatible” for marriage. It is about understanding how inherited haemoglobin variants can be passed from biological parents to their children, and whether a particular combination creates a chance of a significant haemoglobin disorder such as sickle cell disease.
Some combinations carry no risk of an inherited haemoglobin disorder. Others can produce a child with HbSS sickle cell anaemia or HbSC sickle cell disease. A few can produce HbCC, which is a different and generally milder condition. That is why a simple “safe” or “unsafe” label can be misleading.
This UK-focused guide explains what each combination means for your children, how NHS screening and testing work, when to consider testing before pregnancy, what prenatal diagnosis can and cannot tell you, and where to find genetic counselling.
Important: This article is for general education and does not replace advice from a GP, midwife, genetic counsellor or haematologist. If you or your partner has a known haemoglobin variant, or if you are already pregnant, speak to your NHS team about your individual situation.
When people search for “genotype compatibility,” they are usually asking a practical question:
“What haemoglobin genotype could our child inherit, given my genotype and my partner’s genotype?”
This is not about personality, culture or the relationship itself. It is about genetic inheritance — specifically, the genes that determine the type of haemoglobin in your red blood cells.
You inherit two copies of the haemoglobin beta gene, one from each biological parent. Those copies can be:
Your genotype is the combination you inherit:
Carriers do not have sickle cell disease. People with AS or AC are generally healthy and often do not know their carrier status until they are tested.
If you would like a fuller explanation of what each letter means before reading the combinations below, see our companion guide: What is a blood genotype? AA, AS, AC and SS explained.
Genotype matters before marriage or pregnancy because a person’s haemoglobin genotype can affect the possible haemoglobin conditions their future biological children could inherit.
This is particularly relevant when one or both partners carry an unusual haemoglobin gene.
For example, a person with AS has sickle cell trait, not sickle cell disease. They are generally healthy but can pass the HbS gene to their children. If their partner also carries HbS, there is a chance in every pregnancy of a child inheriting two HbS genes and having sickle cell anaemia.
The same principle applies to HbC. A person with AC carries haemoglobin C. HbC is not the same variant as HbS, so AC is not the same as AS. However, if one biological parent has AS and the other has AC, a child can inherit both HbS and HbC and have HbSC sickle cell disease.
This is why checking both partners’ results is more informative than looking at one genotype in isolation, and why a chart that lumps all “unusual” haemoglobin genes together can be misleading.
The following table uses standard Mendelian inheritance for the specific haemoglobin variants shown. All probabilities are per pregnancy and are independent of previous pregnancies.
| Parent combination | Possible child genotypes | Sickle cell disease possible? | Key interpretation |
|---|---|---|---|
| AA + AA | 100% AA | No | No HbS or HbC inherited from these parents |
| AA + AS | 50% AA, 50% AS | No | Children may be HbS carriers |
| AA + AC | 50% AA, 50% AC | No | Children may be HbC carriers |
| AA + SS | 100% AS | No | All children are HbS carriers |
| AS + AS | 25% AA, 50% AS, 25% SS | Yes — 25% HbSS per pregnancy | Classic 1-in-4 sickle cell disease risk |
| AS + AC | 25% AA, 25% AS, 25% AC, 25% SC | Yes — 25% HbSC per pregnancy | HbSC is a form of sickle cell disease |
| AS + SS | 50% AS, 50% SS | Yes — 50% HbSS per pregnancy | Every pregnancy has a 1-in-2 chance |
| AC + AC | 25% AA, 50% AC, 25% CC | No sickle cell disease; 25% HbCC | HbCC is usually mild — not the same as sickle cell disease |
| AC + SS | 50% AS, 50% SC | Yes — 50% HbSC per pregnancy | No AA outcome possible |
| SS + SS | 100% SS | Yes — 100% HbSS | Every child will inherit sickle cell anaemia |
These percentages apply to each pregnancy separately. Two AS parents whose first child is AA still have a 25% chance of an SS child in the next pregnancy. Genetics does not “balance out” across a family. This is the most commonly misunderstood part of genotype compatibility online, so it is worth reading twice.
The table also assumes only the classic HbA, HbS and HbC genes are involved. If either partner also carries beta-thalassaemia trait or another haemoglobin variant, the simple combinations above do not fully apply. NHS antenatal screening in England tests for beta-thalassaemia in all pregnant women alongside sickle cell, precisely because a thalassaemia gene inherited with HbS can cause sickle beta-thalassaemia, which is a form of sickle cell disease. If either of you has a result mentioning thalassaemia, ask for interpretation by a genetic counsellor rather than relying on the table above.
AA + AA produces AA children only. No HbS or HbC is inherited from either parent.
AA + AS produces a 50% chance of AA and 50% chance of AS in each pregnancy. No child can have sickle cell disease from this pairing, because only one parent carries HbS. Some children will be healthy carriers.
AA + AC works the same way for HbC. Children have a 50% chance of AA and 50% chance of AC. None will have HbSS, HbSC or HbCC.
AA + SS is often misunderstood. The AA parent can only pass HbA. The SS parent can only pass HbS. So every child inherits one HbA and one HbS and will be AS — a carrier. No child from this pairing will have sickle cell disease themselves, although they will be able to pass HbS to their own children later. The parent with SS has sickle cell disease and will have their own specialist care needs, but that does not change the inheritance calculation for the children.
Two AS parents each carry one HbA and one HbS. The possible outcomes in each pregnancy are:
This is the classic 1-in-4 risk that NHS guidance discusses when both parents are sickle cell carriers. The 25% applies to every pregnancy. Two AS parents could have four unaffected children in a row, or an affected child on the first pregnancy — the probability resets each time.
Couples in this situation are routinely offered genetic counselling and, during pregnancy, the option of prenatal diagnosis.
This is the combination most often oversimplified online. It is not the same as AS + AS.
The AS parent can pass HbA or HbS. The AC parent can pass HbA or HbC. The possible outcomes are:
HbSC is a recognised form of sickle cell disease. It is generally milder on average than HbSS, but it still causes sickling and has its own complication profile — retinopathy and avascular necrosis are more common in HbSC than in HbSS, for example. It is not something to dismiss as “carrier-like.”
If you and your partner are AS and AC, genetic counselling is particularly worthwhile because generic online charts often get this pairing wrong.
The AS parent can pass HbA or HbS. The SS parent can only pass HbS. So each pregnancy carries:
There is no AA outcome because the SS parent has no HbA to pass on. This is a higher-risk combination and specialist advice is important.
Two AC parents can produce:
HbCC (haemoglobin C disease) is not the same as sickle cell disease. It is usually mild — often causing only a mild anaemia, sometimes an enlarged spleen or gallstones. Many people with HbCC live without significant symptoms and may not need active treatment.
This is an important distinction because some online compatibility charts label AC + AC as “high risk” in the same category as AS + AS. Medically, the outcomes are different, and the article you read should reflect that.
The AC parent can pass HbA or HbC. The SS parent can only pass HbS. The possible outcomes are:
There is no AA outcome and no HbSS outcome from this specific pairing. Every child will either be a carrier or have HbSC.
If both parents have HbSS, they can only pass HbS. Every child will inherit HbSS. Both parents already have sickle cell disease themselves and will be under specialist care, and any pregnancy in this situation is managed by a specialist team.
This question comes up in almost every genotype search, so it is worth answering directly rather than avoiding it.
From a strict inheritance standpoint, AA carries the lowest risk of passing on sickle cell disease to children, because it has no HbS and no HbC to contribute. That is a factual statement about inheritance.
It is not a statement about human worth, health as an individual, or who anyone should marry. People with AS and AC are healthy carriers who live normal lives. People with sickle cell disease live full lives with appropriate care. The purpose of genotype information is to help couples understand what could be inherited by their children — not to rank people or dictate relationships.
A better framing than “which genotype is best” is:
“Given my genotype and my partner’s genotype, what could our children inherit, and what are our options?”
That question has a clear medical answer. The marriage question belongs to the individuals involved.
No. This is one of the most common confusions in genotype searches and it is worth clearing up in a sentence.
Haemoglobin genotype (AA, AS, AC, SS, SC) refers to inherited haemoglobin genes and is relevant to sickle cell disease and related conditions.
Blood group (A, B, AB, O with RhD positive or negative) refers to antigens on red blood cells and is relevant to blood transfusion and some pregnancy issues such as RhD incompatibility.
They are inherited separately, tested separately and mean different things. Someone who is O positive could be AA, AS, AC or any other genotype — the two do not predict each other.
If you would like a fuller comparison, our what is a blood genotype guide covers this in more depth. If you specifically want to check your ABO and RhD blood group, that is a separate test — see our blood group testing service.
No. Your haemoglobin genotype is determined at conception by the genes you inherit and is fixed for life. It does not change with diet, age, lifestyle, marriage or pregnancy.
There is one narrow medical exception — a bone marrow or stem cell transplant can change the haemoglobin a person produces, and gene therapy for sickle cell disease is now available in some NHS specialist centres. Neither is relevant to genotype compatibility for marriage, but it is worth knowing that this is the only situation in which someone’s haemoglobin picture can meaningfully change.
If two of your test results appear to disagree, the answer is usually a laboratory interpretation issue (for example, a recent blood transfusion can temporarily affect results) rather than a real change in genotype. Ask the laboratory or a clinician to review both results.
There are three main routes to a haemoglobin genotype test in the UK.
Through your GP. You can ask your GP for a haemoglobinopathy test, particularly if you have a family history, are planning a pregnancy, or have any concern about your carrier status. Availability and referral practice varies by area.
Through NHS antenatal screening. In England, all pregnant women are offered blood testing for haemoglobin variants and thalassaemia as part of the NHS Sickle Cell and Thalassaemia Screening Programme, ideally by 10 weeks of pregnancy. If the pregnant person is identified as a carrier, the baby’s biological father is offered testing too. Scotland, Wales and Northern Ireland have broadly similar screening arrangements — check with your local NHS service for the exact pathway in your nation.
Through a private test. The NHS pathway is built around pregnancy, which means couples who want to know their status before conceiving — rather than reacting once pregnant — sometimes choose to test privately. This is the most honest use case for private testing: it gives you time to discuss results, seek genetic counselling if needed and make informed decisions before pregnancy, rather than compressing all of that into the first trimester.
Our genetic and genotype blood testing service offers this as a preconception option.
Yes — but only through diagnostic testing, and only when there is a known reason to test.
This is where a lot of online content gets confusing, so it is worth being clear about the difference between two things:
Screening tells you about the parents’ carrier status. NHS antenatal screening checks the pregnant person, and the biological father if the mother is a carrier. It estimates the chance of the baby being affected but does not diagnose the baby.
Diagnostic testing tests the baby directly. It is offered when both parents are known or suspected carriers and the couple wants to know the baby’s genotype.
The two diagnostic options in the UK are:
Chorionic villus sampling (CVS) — a small sample of placental tissue, usually taken from around 11 to 14 weeks of pregnancy.
Amniocentesis — a small sample of amniotic fluid, usually taken from around 15 to 20 weeks.
Both are invasive procedures and carry a small risk of miscarriage (the NHS quotes approximately 0.5% or lower, depending on the procedure and operator). They are offered — not required — and the decision is a personal one, usually taken with support from a specialist midwife or genetic counsellor.
Do not rely on claims that a routine pregnancy blood test or standard non-invasive prenatal test can definitively tell you the baby’s sickle cell genotype in NHS care. That is not currently standard practice.
After birth, the NHS newborn blood spot screening programme also identifies sickle cell disease and some carrier states in every baby born in the UK.
If NHS screening identifies both parents as carriers of relevant haemoglobin variants, the specialist team will:
If a diagnosis of sickle cell disease is made prenatally, care planning can begin before birth so that specialist paediatric haematology support is in place from day one.
The purpose of this pathway is informed choice, not pressure in any direction. Couples make different decisions based on their own values, and NHS counselling is designed to support that rather than steer it.
There is no legal requirement in the UK to test genotype before marriage, unlike in some other countries where premarital screening is mandated by law or by community programmes.
Whether testing is worthwhile for you depends on your situation. It is particularly useful if:
If none of the above apply and you have no specific concern, NHS antenatal screening in pregnancy will pick up carrier status routinely. But if you want to plan proactively, preconception testing gives you more time and more choices. Genotype testing also pairs naturally with broader preconception planning — our fertility and pre-conception blood test covers hormone markers such as FSH, LH and AMH for couples thinking about the wider picture before trying to conceive.
What is genotype compatibility?
Genotype compatibility means comparing two biological parents’ haemoglobin genotypes to understand what haemoglobin combinations their children could inherit, and whether any of those combinations would cause sickle cell disease or a related haemoglobin disorder. It is an inheritance assessment, not a verdict on the relationship.
Can AA marry AS?
Yes. Children will be 50% AA and 50% AS. No child from this pairing can have sickle cell disease.
Can AA marry SS?
Yes. Every child will be AS — a carrier. None will have sickle cell disease.
Can AS marry AS?
Yes. Each pregnancy carries a 25% chance of AA, 50% chance of AS and 25% chance of HbSS sickle cell anaemia. Genetic counselling is recommended.
Can AS marry AC?
Yes. Each pregnancy carries a 25% chance of HbSC, which is a form of sickle cell disease. This combination is often oversimplified online — HbSC is real and clinically significant, generally milder than HbSS but with its own complications.
Can AS marry SS?
Yes. Each pregnancy carries a 50% chance of AS and 50% chance of HbSS sickle cell anaemia.
Can AC marry AC?
Yes. Each pregnancy carries a 25% chance of HbCC. HbCC is generally mild and is not the same as sickle cell disease. This combination should still be discussed with a healthcare professional but does not carry the same risk profile as AS + AS.
Can AC marry SS?
Yes. Each pregnancy carries a 50% chance of AS and 50% chance of HbSC sickle cell disease. There is no AA or HbSS outcome from this specific pairing.
If our first child is AA, does that lower the risk for the next pregnancy?
No. Each pregnancy is an independent genetic event. Two AS parents whose first child is AA still have exactly a 25% chance of an SS child in the next pregnancy. This is the most commonly misunderstood aspect of genotype compatibility.
Which genotype is the best?
AA carries the lowest risk of passing on sickle cell disease because it has no HbS or HbC. That is a factual point about inheritance, not a statement that AA is a healthier or “better” genotype for the individual.
Can genotype change?
No, except through bone marrow or stem cell transplant or gene therapy, which are specialist medical treatments and not relevant to marriage planning.
Is genotype the same as blood group?
No. Blood group (A, B, AB, O with RhD) and haemoglobin genotype are inherited and tested separately. Someone who is O positive could be any haemoglobin genotype.
Can I know my unborn baby’s genotype?
Yes, through prenatal diagnostic testing — CVS from around 11 to 14 weeks or amniocentesis from around 15 to 20 weeks. Both are invasive and carry a small miscarriage risk. They are offered when both parents are known or suspected carriers.
What’s the difference between screening and diagnosis in pregnancy?
Screening checks the parents’ carrier status and estimates the chance of the baby being affected. Diagnosis tests the baby directly. NHS antenatal screening is universal in England for pregnant women; diagnostic testing (CVS or amniocentesis) is only offered when screening indicates a real risk.
Does AS mean I have sickle cell disease?
No. AS is sickle cell trait — carrier status. People with AS are generally healthy and asymptomatic. The relevance of AS is what can be passed to children, not the carrier’s own health.
How accurate is a genotype compatibility test?
Laboratory-based haemoglobin analysis (typically HPLC or electrophoresis) is highly accurate. Results are usually definitive. Rapid or home tests can be less reliable and should be confirmed by a proper laboratory if there is any doubt.
What happens if we are already married and find out our combination is high-risk?
Nothing changes about your marriage. Options include genetic counselling, prenatal diagnosis during pregnancy, and — for some couples — IVF with pre-implantation genetic testing to select embryos without the condition. NHS specialist services can discuss all of these without pressure in any direction.
Speak to your GP, midwife or a genetic counsellor if:
If you would like to book a genotype test at one of our Essex clinics, you can see all Swift Blood Test locations including Billericay, Basildon, Chelmsford, Maldon, Thurrock, Brentwood and Bromley.
Support and information are also available from the Sickle Cell Society, the UK charity that works alongside the NHS specialist services.
