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.
Sickle Cell Disease is an inherited blood disease that affects haemoglobin.
Haemoglobin (Hb) is a protein found on 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) which determine what you or I become 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:
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.
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%.
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