I’m paraphrasing this question, but someone wanted to know the chromosomal “logistics” of Down Syndrome, so to speak.
A typical person has 46 chromosomes; from each parent, we get one copy of chromosomes numbered 1-22, plus a sex chromosome. In the vast majority of cases (about 95%), a child with Down Syndrome has one extra copy of the 21st chromosome. The chromosomal difference occurs at conception. We refer to this as Trisomy 21 – three copies instead of two. This is why World Down Syndrome Day is on March 21 – 3/21 – get it? 😄 We wear wacky or mismatched socks in WDSD because chromosomes look kind of like socks, and on WDSD we celebrate and embrace the differences that come along with not having quite the same “socks” as everyone else.
About 3-4% of children with Down Syndrome have “translocation Down Syndrome,” which means that the third copy of chromosome 21 is attached to a different chromosome. In some cases, this translocation is inherited from a parent who is a “balanced carrier” – they have two copies of chromosome 21, but one of them is attached to another chromosome. The chances of having a child with Down Syndrome increase in these cases, as the child may inherit both the one copy of the 21st chromosome that’s in the right place and the extra copy attached to the other chromosome. I think that’s roughly how it works – don’t quote me on this. It gets a lot more complicated, but this is the basic gist of it. If a child has translocation DS, the parents should be tested to determine whether they are a carrier.
In the remainder of cases (about 2%), the chromosomal difference occurs at a later stage. A typical zygote is conceived, but during the process of cell division, some of the cells wind up with an extra copy of the 21st chromosome and some do not. The two lines of cells divide into more cells like themselves, until you wind up with an adorable little baby who has some percentage of cells in their body with three 21s, and some percentage with typical chromosomes. We call this “Mosaic Down Syndrome.” A common misconception is that a child with mosaic DS is not as impaired as a child with full trisomy 21, or that a child who is less impaired must necessarily have mosaic Down Syndrome. The reality is that Down Syndrome involves a wide range of potential cognitive and medical issues, and in any case of Down Syndrome, some of them will be present and some will not. The impact of the extra chromosome on a person with mosaic Down Syndrome depends on a number of factors, including the percentage of cells involved and where those cells are located.
Have I left anything out?
