What IS Down Syndrome? Part II of III
Last week we talked about what Down syndrome is. To give a quick recap - Down syndrome occurs as a result of an extra third (partial or full) copy of the 21st chromosome, resulting in an individual having 47 chromosomes instead of the typical 46. Down syndrome is the most commonly occurring chromosomal disorder, with approximately 6,000 babies born with Ds every year in the United States. We also learned that there are different types of Down syndrome — trisomy 21, mosaicism, and translocation — and that is where we’ll go more in depth today.
Trisomy 21 is the most commonly occurring form of Down syndrome, accounting for approximately 95% of cases; followed by translocation at about 4%; and then mosaicism, which accounts for just 1% of cases. Trisomy 21, also called nondisjunction, occurs prior to conception when a pair of 21st chromosomes either from the egg or sperm, fail to separate. As a result, when the embryo begins to develop the extra copy of chromosome 21 is replicated in every cell.
Mosaic Down syndrome is interesting because not every cell has the third copy of chromosome 21, resulting in some cells that have the typical 46 chromosomes and some that have 47. Mosaicism can happen in one of two ways: as you saw with Trisomy 21, the embryo could initially have 3 copies of chromosome 21, but somewhere during cell division one or more cells lost the extra copy. OR the initial embryo has only two copies of chromosome 21, but for an unexplained reason during cell division, one copy of chromosome 21 is duplicated and from that point on it continues to divide by ordinary cell division, just with an extra copy.
Translocation occurs during cell division when a part of chromosome 21 breaks off and attaches to another chromosome, usually #14. Translocation is the only type of Ds that can be genetically passed down from either the mother or the father, but this is very rare and occurs in only 1% of all individuals with Down syndrome.
As with many different types of genetic disorders, the likelihood of having another baby with Down syndrome does go up for women who already have one child with Ds, regardless of the age of the mother. The odds go to about a 1 in 100 chance of having another baby with Trisomy 21 until the age of 40, at which point the odds increase again. If the type of Down syndrome is inherited translocation, then the odds of it recurring are anywhere from 3%-15%.
Come back next week to continue reading our education mini series! We’ll dive into the ways that extra little chromosome makes the body tick differently, and cover the different types of conditions and disorders that are commonly associated with Down syndrome.
And P.S. — in case you were wondering which type of Down syndrome Luke has, can you believe that even we still don’t know? His genetic karyotype doesn’t read clearly and the company that did the testing of course won’t give any clarification to me unless I shell out money for a consultation. So, we’ll be waiting to figure all of that out until we meet with his genetic doctor in JULY (since that’s the earliest they could get him in). Sigh — the lack of answers and frustration continues! But honestly, whatever type he has doesn’t matter to us. He’s Luke, and he’s just so stinkin’ cute.