Chimeras, Mosaics, and Other Fun Stuff

My daughter had a bone marrow transplant when she was 16. The donor was male. She is now 30 and is pregnant. She just had some blood work done, and the lab called to say that the blood sample they sent in was male, not female! My daughter is now worried about the baby looking like the donor instead of her, and/or having medical issues. Does she need to worry about this?

- A concerned adult from North Carolina

March 5, 2019

Rest assured the baby will not look like the donor! This is due to the fact that the baby will inherit DNA from Mom, not the donor. 

Let’s look into what it means to have a bone marrow transplant. 

What happens when you have a bone marrow transplant? 

To begin, the cells in your body that create blood cells (bone marrow) are destroyed by chemotherapy or radiation. 

After all your own bone marrow cells are gone, you’re ready for the transplant. Bone marrow cells are harvested from the donor and injected into you. These new cells make their way into your own bone marrow, where they take up residence.

Modified from image1 and image2

From that point on, your bone marrow is made up of cells from the donor. These cells do exactly what they’re supposed to: create new blood cells. 

This is a type of stem cell therapy. The bone marrow contains stem cells, which just means they’re cells that can keep producing new blood cells indefinitely. They won’t die off, and your body won’t replace them. You’ll permanently have bone marrow cells from the donor.

This means all future blood cells contain the donor’s DNA. 

But remember we have lots of different types of cells including skin, hair, muscle cells and more. Only the blood cells will have donor DNA – every other cell in the body will have the original DNA. 

Another word for having DNA from two people is chimera. For more on chimeras, check out this previous article.

People who have had stem cell transplants should consider using different sources of DNA for genetic testing. One possible substitute could be using skin cells from your cheeks (saliva samples) instead.  

Could the baby get any DNA from the donor? 

You may remember that babies are created using Mom’s egg cells and Dad’s sperm cells. Since the donor’s DNA is only present in the blood-related cells, the egg will just have Mom’s own DNA. 

That means the baby will inherit his/her DNA from Mom, not the donor. Half of his/her DNA will be from Mom, and half will be from Dad. And since our DNA plays a role in what we look like, the baby will probably look a lot like Mom and Dad! 

After conception, mom and baby don’t really swap cells (or DNA). The baby receives all its nutrients through the placenta, which mostly keeps everything separate.

Sometimes a few cells from mom may slip through the placenta into the baby. Or a few of the baby’s cells may go into mom! But even if a few blood cells do enter the baby, they’ll be outnumbered by the baby’s own cells. And they generally die off pretty quickly. 

So it’s theoretically possible that a few blood cells with the donor’s DNA may slip into the baby. But they won’t last very long, and they’ll be vastly outnumbered by the trillions of other cells that are the baby’s own personal mix of Mom and Dad’s DNA. Any rare cells with donor’s DNA won’t affect how the baby looks.

So overall, don’t worry! Your grandbaby will have a unique mix of your daughter’s DNA and the father’s DNA.

By Laura Hayward, Stanford University

Chimeras, Mosaics, and Other Fun Stuff