A lot of people have asked me what it means to be a balanced carrier for Trisomy 21. This particular subject is complex, yet easy to comprehend once you get the main idea. I'll try my best to explain this in a way that won't have you yawning or worse, falling asleep. My husband is a balanced 21/22 carrier for Trisomy 21. Translocation Down syndrome is rare. The 21/22 translocation is very, very rare.
I often get the question, ”Does Down syndrome run in your family?” Of all people, I've been asked this exact question by a nurse! I told you I was going to put the rest all the misconceptions. This is one of those questions that is like nails on a chalkboard for me. First of all, Down syndrome isn’t like twins, as it doesn’t typically “run” in families. 90-95% of the time it’s just an error in cell division. It just happens and we don’t know why. (refer my post a few days ago about the 3 different types of Down syndrome if you’re confused.) The only form that can be(but NOT ALWAYS) hereditary is Translocation Down Syndrome. In our case, it is hereditary. We didn’t know this prior to Austin’s birth.
I also get questions about my husband.
If he’s a balanced carrier, what’s wrong with him?
Other than the fact that he snores at night, nothing. In all seriousness, there is no way to tell if a person is a balanced carrier just by looking at them. Our genetic doctor suspected that Ryan was the carrier before the results came back because his family has a history of miscarriages. Balanced carriers are not associated with any phenotypic abnormalities. In other words, they are clinically normal. Some of their genetic material is “rearranged“ - so to speak. However, since there isn’t any extra genetic material, it doesn’t affect them. There is a risk of unbalanced gametes which leads to miscarriages. Moreover, if the translocation involves chromosome 21, there is a higher chance of having a child with Down syndrome. In our case, my husband’s translocation involved chromosome 21 and 22. Based on this specific translocation, we have an increased risk for miscarriage and our increased risk for Down syndrome in future pregnancies is 5% or 1:20.
Are you going to have more children?
Yes. Just one more. Due to our increased risks, we are going to go the in vitro fertilization route for our next pregnancy.
What other types of balanced Translocations are related to Down syndrome besides 21/22?
13/21, 14/21 and 15/21 are the most common. There is also a 21/21.
Random Fact: The 21/21 Translocation is the only balanced translocation where the child will have a 100% chance of Down syndrome.
The other types come with varying risks that have to do with whether the carrier is the mother or father. The risk for recurrence is higher when the mother is carrier.
Here is a karyotype (example) of a balanced male carrier. This particular male has the 14/21 balanced translocation. I tried to find a karyotype for the 21/22 translocation, but it’s extremely rare and I had no such luck. Look in the lower right corner. Notice how the total chromosome count is: 45. Most of us have 46 chromosomes. Remember: No extra genetic material = doesn't affect them.
I'm a total genetic nerd, I know. Never in my life did I think I'd be writing about chromosomes and you know what? I like it.
That's all. I hope you're all a little more informed. :)