What is Down Syndrome: Genetics
If you are a new parent one of your first questions may be “What is Down syndrome?” (This by the way, was one of my husbands first questions when Noah was born.)
Down syndrome (Trisomy 21) is a chromosomal abnormality where a child is born with three copies of the 21st chromosome (Just like in the picture!)
Chromosomes hold our genetic information, and there are 23 types of chromosomes. One set of those being your sex chromosomes (X, and Y), and the others labeled 1 through 22. When babies are created, they receive 23 chromosomes from their mom contained in her egg and 23 chromosomes from their dad contained in his sperm, totaling 46 chromosomes.
Children with Down syndrome are born with an extra 21st chromosome.
Three Types Of Down Syndrome:
1. Nondisjunction: This is the most common reason. Cells containing 46 chromosomes are split equally to create eggs and sperm, which then contain 23 chromosomes. Nondisjunction means that the cells did not equally divide the genetic information. This creates a sperm or egg that contains 24 chromosomes with two copies of the 21st chromosome. That egg/sperm joins with a typical egg/sperm with 23 chromosomes making a cell that has 47 chromosomes and three copies of the 21st chromosome. This is known as Trisomy 21. This is a random occurrence.
2. Translocation: The egg or sperm have 23 chromosomes, however part of the 21st chromosome has attached itself to one of the other chromosomes during the division processes that occur when the egg or sperm were made. Therefore, even though they have the right number of chromosomes there is actually extra 21st chromosome material. This CAN be (not always) an inherited condition, which can increase your risk of having another child with Down syndrome. Your doctor will want both parents to be tested to see if this is a possibility.
3. Mosaic: Not all of the cells are affected, so the baby would have cells with 46 chromosomes and also 47. This can occur in two ways. The baby either started with 46 chromosomes, however nondisjunction occurred with further cell division in only part of the cell lines creating some cells with three copies of the 21st chromosome. The other way is that the baby started out as trisomy 21 and nondisjunction occurred creating cells that now contain 46 chromosomes. This is a random occurrence.
This is a very basic overview, but hopefully helps you understand that a child born with down syndrome isn’t sick or suffering. He or she simply has an extra copy of their 21st chromosome.
Yes, even a blog has to have fine print these days. Here is ours: disclaimer: This post was written by my amazing wife, and the mother of a child diagnosed with Down Syndrome! She is also a pediatrician, but not your pediatrician! All of the information on this website (although great!) is for informational purposes only. Nothing on this site is intended to create a physician-patient relationship, replace the services of a licensed, trained physician, or to be a substitute for medical advice of a physician or trained health professional licensed in your state/Country. You should not rely on any of the information contained on this website, and should contact a physician licensed in your state/Country on all matters relating to your (or someone else’s) health. You agree that you shall not make any health or medical related decisions based in whole or in part of anything contained on this website.