Can someone be born with the sickle cell trait from a white mother who has Rh- blood?
-A curious adult from North Carolina
March 29, 2017
First, thank you for your question! I think what you might be really getting at is whether someone can inherit sickle cell trait from a white mother who has sickle cell trait and Rh- blood.
And the short answer to this question is yes they can! There are almost certainly people with sickle cell trait who inherited it from Caucasian mothers with Rh- blood and sickle cell trait.
Now this doesn’t mean it is necessarily common.
Sickle cell trait is somewhat rare in people of European origin. In fact, only 1 in 600 or so Caucasians have it (compared to 1 in 14 African Americans).
And Rh- blood is not very common either. Only around 1 in 6 people of European descent have Rh- blood.
But, since the two traits aren’t connected, it is definitely possible for a mother to have sickle cell trait and Rh- blood. This also means it is possible for her to pass sickle cell trait on to her child(ren).
Let’s explore the reasoning behind this answer a bit more!
Having Rh- blood and having sickle cell trait aren’t connected. In genetics terms, we say that the two traits are unlinked.
This means that they are inherited independently of each other. And actually, aside from both affecting blood properties, sickle cell trait and Rh blood type don’t have much to do with each other.
So, if a woman has Rh- blood and sickle cell trait, then she can pass either to her child(ren).
An easier (and more fun!) way to think about this is with candy. Say you have three bowls of candy.
The first bowl has two kinds of bubblegum, blueberry and cherry. The second bowl has two kinds of lollipops, watermelon and lemon. And the third bowl has one kind of blowpop (lollipops with a bubblegum center), a lemon lollipop with a blueberry bubblegum center.
Now let’s say you want a lemon lollipop and blueberry bubblegum. And you have to either pick a piece of bubblegum and a lollipop from the first two bowls or a blowpop from the third. Blindfolded.
In this case your best bet is bowl 3. Because the two candies are linked in that bowl, you’ll get the right combination each time.
If you choose to pick from bowls 1 and 2 instead, then you have a 1 in 2 chance of getting the right lollipop and a 1 in 2 chance of getting the right bubblegum. Here, your chances of getting both drop to 1 in 4.
Now of course if you want a lemon lollipop and cherry bubblegum, you have to go with bowls 1 and 2. Bowl 3 doesn’t have that combination.
Rh- blood and the sickle cell trait are more like choosing from bowls 1 and 2. The two traits are unlinked.
This means that a child of a Caucasian woman with Rh- blood and sickle cell trait could inherit both, one, or neither traits from her. The two do not travel as a pair—they are not a blowpop.
Of course the instructions for Rh- blood and sickle cell trait are not found on pieces of candy. Instead, they are found on chromosomes.
DNA, Traits, and Chromosomes
In genetics, a trait refers to any genetically determined characteristic. Sickle cell trait and Rh- blood are both examples of traits.
DNA is the molecule our bodies use to spell out the instructions needed to make each of our traits. Most peoples’ instructions are found in 46 long pieces of DNA called chromosomes, which are arranged into 23 pairs.
As humans, we get one member of each chromosome pair from our mom and one member of each pair from our dad. Similarly, when we go to have children, we pass on one member of each chromosome pair and our partner passes on the other member.
This explains why children are a mixture of their parents. And as you’ll see later, why our hypothetical mother can give sickle cell trait to her child(ren).
Here is a picture of those chromosomes, or units of DNA, we have been talking about:
Looking at our diagram, this person inherited one member of each chromosome pair from their “pink” parent, and one member of each chromosome pair from their “blue” parent.
As we said earlier, when this person has children, they will pass on one member of each pair and their partner will pass on the other.
This means that for each chromosome they can either pass on their “pink” copy, or their “blue” copy.
Whether they pass on their “pink” or their “blue” member of each chromosome pair occurs by chance. And the color passed on is independent for each chromosome.
So a person might pass on their “pink” copy of chromosome 1, their “blue” copy of chromosome 2, their “blue” copy of chromosome 3, etc.
Now, I just want to make one last point, before we can finish answering your questions.
What Causes Sickle Cell Trait and Rh- Blood?
Earlier we said that sickle cell trait and Rh- blood are both traits, and that DNA is the molecule our bodies use to spell out the instructions needed to make each of these traits.
So, our bodies have a specific set of instructions that code for sickle cell trait and a specific set of instructions that code for Rh blood type. Spelling mistakes in these trait instructions are called mutations.
If you have sickle cell trait, it is because you have a mutation in the instructions for a molecule in blood called hemoglobin on one of your copies of chromosome 11.
And if you have Rh- blood, then it is because you have a spelling change in the instructions for Rh blood type on both of your copies of chromosome 1.
And with that, I think we are ready to tie everything we have learned together to answer your question!
So, I alluded to this earlier, but the key to your question is that the instructions for Rh- blood are located on a different chromosome than the instructions for sickle cell.
In other words, the traits are located on different types of candy and are therefore in different bowls!
This means someone can get both, one, or neither. It all depends on which their parent happens to pass down to them.
So for your question, it is possible for a Caucasian mother to have inherited a chromosome 11 with sickle cell trait from one parent and a chromosome 1 with Rh- from both her parents (remember you need for both copies to have spelling changes to end up with Rh- blood).
Let’s draw this out our mom’s chromosomes:
Remember that the Rh gene is on chromosome 1 and that to have Rh- blood, both copies need to have a mutation. I have shown this with an X on each chromosome 1.
Sickle cell trait happens when just one copy of the hemoglobin gene has a mutation. That is why just one chromosome 11 has an X.
Now let’s see the odds for her kids. In this example, we will ignore dad’s DNA and just focus on mom’s and show dad’s with dashed lines.
Since mom only has mutated copies of her chromosome 1 with the Rh- trait, in each pregnancy she will always pass on a copy with an X, as is shown in the diagram above. Both potential kids have a pink chromosome 1 with an X.
Looking at the diagram, we also see that because mom has sickle cell trait, she has one copy of her chromosome 11 with an X. Therefore, each child has a 1 in 2 chance of getting the chromosome 11 with the X.
Combing these events, we can see that the probability of getting a chromosome 1 with an X and a chromosome 11 with an X is ½ (1 x ½ = ½). I circled that possibility in yellow on the diagram for you.
Going back to your original question, this means that the probability of someone inheriting sickle cell trait from a white mother who has sickle cell trait and Rh- blood is ½.
Thanks again for writing in! I hope that was helpful!
By Megan Nathan, Stanford University