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How To Know the Chance of SS Genotype with AS Parents

June 10, 2020

A common question is: “What’s the chance of SS genotype with SS parents”?

Updated March 2022

We break down the chances of having children with Sickle Cell Anaemia (SCA) when both parents are carriers of the condition, i.e. trait. It is a little technical, but if you want to understand how sickle cells are transmitted, read on.

Sickle red blood cells on a black background

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 Anaemia?

Sickle Cell Anaemia 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 SCA 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 SCA is the Hb S gene. It is carried on the non-sex chromosomes.

How is the SS Gene inherited?

 SCA 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 SCA has inherited 1 abnormal S gene from Mum and 1 abnormal S gene from Dad.

Having 2 abnormal SS Genes gives them the condition Sickle Cell Anaemia, SCA.

How the SS Gene Causes Illness

But what is the problem with SCA?

Essentially having the 2 SS genes means that they form an abnormal type of haemoglobin.

This is called 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 SCA

When it comes to blood diseases, there are different gene combinations that lead to different health conditions, including SCA (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 mutations.

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 SS genotype from 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.

The chance of SS genotype is that in each pregnancy, the risk of the child you bear having SCD is 25%, while their risk of being a carrier is 50%.

Finally, the risk they do not carry the sickle gene or express the disease is 25%.

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


All AskAwayHealth articles are written by practising Medical Practitioners on a wide range of healthcare conditions to provide evidence-based guidance and to help promote quality healthcare. 

The advice in our material is not meant to replace the management of your specific condition by a qualified healthcare practitioner.
To discuss your condition, please contact a health practitioner or reach us directly

Photo by Aakash Dhage on Unsplash

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