Chance of SS genotype with AS parents?
Updated March 2022

A common question is: “what’s the chance of SS genotype with SS parents”?
We break down the chances of having children with Sickle Cell Disease (SCD) when both parents are carriers of the condition ie trait. It is a little technical, but if you want to understand how sickle cells are transmitted, read on.
Sickle Cell Disease is an inherited blood disease that affects haemoglobin.
Haemoglobin (Hb) is a protein that exists in our red blood cells that carries oxygen – and we need it to survive.
What Causes Sickle Cell Disease?
Sickle Cell Disease is not caused by an infection or vaccination or curse – it has been clearly proven to be a genetic condition.
This means you can only ever get SCD because you were born with 2 genes for the disease that you received from both parents.
Genes carry material (DNA) that determines what you or I become. You will find these DNA materials or sequences on chromosomes.
We have mainly 2 types of chromosomes – sex and non-sex chromosomes.
The sex chromosomes determine the sex that we become, while the non-sex chromosomes determine everything else.
The gene of concern in SCD is the Hb S gene. It is carried on the non-sex chromosomes.
How is SS Inherited?
SCD is an inherited condition called autosomal recessive.
This tells us 2 things – Autosome or non-sex chromosome – means having the disease does not depend on the individual’s sex, and recessive means two abnormal or mutated gene copies must be present.
So, a person who has SCD has inherited 1 abnormal S gene from Mum and 1 abnormal S gene from Dad.
Having 2 abnormal SS Genes gives them the condition, SCD
How SS Causes Illness.
But what is the problem in SCD?
Essentially having the 2 SS genes means that they form an abnormal type of haemoglobin.
This is called the sickle haemoglobin because it changes the shape of the red blood cell which carries haemoglobin – from where the disease gets its name.
Sickle red cells cannot carry oxygen properly because they have lost the normal disc shape of normal red cells – and they are sticky.
This means they can block the blood vessels and affect oxygen concentration – leading to various types of illness such as ‘sickle cell crises’ and may result in death.
Abnormal Genotype Combinations in SCD.
When it comes to blood diseases there are different gene combinations that lead to different health conditions including SCD (for example Thalassemia).
We will focus on the genes for SCD to provide the answers needed for this article.
The haemoglobin gene types we need to know are Hb A (AA), HB AS (AS), and Hb SS (SS).
Hb A is the normal genetic type, that is both gene copies are free of abnormal changes or mutation.
Carrier/ Trait – Hb AS (AS Genotype)
Hb AS genotype is the carrier or trait.
Such a person only has 1 copy of the mutated (abnormal) gene, while the other copy is normal.
So they would have received the mutated gene from one parent and a normal gene from the other.
They typically do not show signs of the condition.
Sickle Cell Anaemia – Hb SS (SS Genotype)
The person with the Hb SS gene has both their gene copies mutated and hence will display the disease.
They will also only transmit abnormal or mutated genes to their kids.
Since each parent has SCD, they each transfer an abnormal gene to each child, they will thus have kids who also have the SCD gene.
How SS or AS is inherited
So as you see – it is a remarkably simple outcome in the chance equation – that the chance of 2 SCD parents having a child with SCD is 100%.
But it is different for people with the trait/ carriers, that is AS, who have kids together.
The relevance is that each AS parent could pass a copy of the abnormal HbS gene to any of their children; or a normal gene at every pregnancy.
This creates three possible scenarios:
Scenario One
Every time a couple who are both AS are pregnant, the chance the child is SS is 25%.
This is what happens if each of them transmits the abnormal gene in that pregnancy.
Scenario Two
On the other hand, if both transfer their normal gene to the pregnancy, the chance the child is AA and free of SCD is also 25%.
Scenario Three
What if one parent passes the mutated gene to the child while the other passes the normal gene?
There is a 50% chance this happens and the result is the child will be genotype AS.
“Preventing SS Genotype” – Know the Risk
So this is what you must know as an AS couple dating about the scenarios when you choose to have children together – that in each pregnancy, the risk of the child you bear having SCD is 25%, while their risk of being a carrier is 50% and finally the risk they do not carry the sickle gene or express the disease is 25%.
Does this explain a little about the inheritance of SCD?
There is a lot to think about as a person with genotype AS who is in a relationship with another person with AS.
Get a copy of our free 7-point checklist to learn other issues you must consider if you are AS genotype.
And, COMMENT BELOW – what would you do – knowing that if you have children with someone you love, there is a 25% risk they may have a serious illness like SCD?
Read More about Sickle Cell Disease here
Editing by AskAwayHealth Team
Disclaimer
All AskAwayHealth articles are written by practising Medical Practitioners on a wide range of health care conditions to provide evidence-based guidance and to help promote quality health care. The advice in our material is not meant to replace the management of your specific condition by a qualified health care practitioner.
To discuss your condition, please contact a health practitioner or reach us directly.
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[…] Chance of SS genotype with AS parents? […]
Please I’m a AS Gene carrier and I’m dating a lady of AS Genotype. Following the theorytical cross breed between these two AS Genotypes, the offsprings will present as follows: AA, AS, AS, SS. Does this occur naturally? Does it normally take this sequence assuming there has been no miscarriage or abortions?
Hi Ojobo, thanks for the q.
First, this is not theoretical.
AA, AS, or SS genotypes are possible outcomes in every pregnancy. The way to look at it is this – whenever you guys get pregnant, the chance the baby has AA is 25%.
The chance the baby is SS is 25%. While the chance the baby is AS is 50%. These are the chances in that pregnancy, the next, or the one after that – regardless of whether the pregnancy goes to term or ends in miscarriage.
There is no sequence. Each pregnancy has those same chances every single time. I provide further clarification and guidance via our Concierge Services.
Best Wishes
Please help me Dr sir, I’m really in love with my spouse… And he loves me the way i am, we loves ourselves but we just found out that he’s AS and i am SS.. We’re afraid to lose each other
My question is
What’s our chances if we later get married?
Hello Olufunke, thanks for writing in and making an enquiry about the SS genotype.
In respect of the chance of having a child with SS genotype, it is 50% every time you are pregnant. Otherwise, your baby can have the AS genotype. In other words, it’s 50%-50% that you can have a child with sickle cell disease (SS) or a sickle cell carrier (AS). This is a significant chance (50%) of having a baby with SS genotype that both of you must be aware of and discuss the implications. Read more about this here
In this post, I share more knowledge and options people may consider when they wish to stay together. Having genetic counselling in the first instance is recommended so you both are clear about the implications, risks and options. Best wishes.
Administrator please help me, my spouse told me that her genotype is AA while I know very well that am As and I told her, after one year of our relationship I made up my mind to marry her. I but to my greatest surprise after the blood tests her results came out As. The love is too much between us. What should I do? Am confused.
Hello, thanks for visiting the page. This post here should help answer the question . Best Wishes to you both.
Hi Doc. I am AS and my fiancé is AS too.. I have had 6 abortions 😔… Please, what are the chances that we won’t give birth to children with only SS genes?
Hello, as mentioned in the post, the chance of SS genotype every time you are pregnant is 25%. Here’s the link if you want to enquire further.