31 for 21, Day 5: Mosaic Down syndrome

Question:   On Reece's Rainbow, I read that it was thought P has Mosaic Down Syndrome. How is this different from the other types of Down Syndrome? Is it a milder form? 


Excellent question!!  As most of you know, Down syndrome, or Trisomy 21, is when there is an extra copy of the 21st chromosome.  With your standard run of the mill Down syndrome that 95% of those with DS have, just like our Evan- the extra chromosome failed to separate before or at conception on either the sperm or the egg.  As the embryo grows and cells replicate, the extra chromosome is present in every cell of the body.  


In 1% of the cases, the nondisjunction of chromosome 21 occurs after fertilization- after cells already started replicating.  There is now a mixture of cells with 46 chromosomes and cells with 47.  This is mosaic Down syndrome.  Translocation Down syndrome is the third type and accounts for the remaining 4%.  With Translocation DS, part of the 21st chromosome breaks off and attaches to a different chromosome, usually number 14.  You can read more about what causes the three types of Down syndrome at the NDSS site HERE. 


Some may consider mosaic Down syndrome to be "milder," but in reality there haven't been many studies conducted comparing the two.  From ds-health.com:


At the present time, there is not much research on the similarities and differences between simple trisomy 21 and mosaic trisomy 21. One report published in 1991 on mental development in Down syndrome mosaicism compared 30 children with mosaic Down syndrome with 30 children with typical Down syndrome. IQ testing showed that the mean IQ of the mosaic group was 12 points higher than the mean of the non-mosaic group. However, some children with typical Down syndrome did score higher on the IQ tests than some of the children with mosaic Down syndrome.

The Department of Human Genetics at the Medical College of Virginia has had an ongoing study project of children with mosaic DS. In a survey of 45 children with mosaicism, they found that these children did show delayed development compared to their siblings. When 28 of these children with mosaicism were matched up with 28 children with typical Down syndrome for age and gender, the children with mosaicism reached certain motor milestones earlier than children with typical DS, such as crawling and walking alone. However, the speech development was equally delayed in both groups.

I believe the assumptions that P has mosaic Down syndrome are made by looking at her photo and not seeing many of the common facial features of children with DS, and also from the description of her being "high functioning."  We have not received any of her medical records, so we do not know her specific diagnosis or how that diagnosis was made.  Down syndrome and mosaic Down syndrome are diagnosed by blood tests, and these tests are expensive.  We don't know when P's birth parents turned her over to the orphanage, and if it was at birth- we don't know if there would be money to run this test on an orphan. We intend to ask the orphanage director about how her diagnosis was made when we arrive for our first visit.  Regardless, once we bring her home we will have the test run again along with all the other bloodwork she will have taken.  


Thanks so much for asking!  Please keep the questions coming!  I also answered a question today on our family blog regarding my two cents about "high-functioning" so be sure to check it out HERE! :)