What Is Down syndrome? The 21st Chromosome Simply Explained

[This post is a part of the “New Parent Guide To Down Syndrome” blog series. Be sure to check out the other posts!]
down syndrome chromosone chart

Not two, but three 21st chromosomes!

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.

[This post is a part of the “New Parent Guide To Down Syndrome” blog series. Be sure to check out the other posts, and subscribe so you can stay updated!

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.

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  1. I am seriously IN LOVE. In love with Noah and with this blog- I am so excited to have found it!!

    • Patti, thank you so much! I look forward to getting to know you and your family as well! Please keep stopping by.

      By the way…you should grab a Gravatar —–> http://en.gravatar.com/

    • ali rehman says

      great i have got more help from this explination of down syndrom and feel so exited when found this post, it is explained in so easy way.
      Thanks to Noah’s DAD

  2. We actually were given a copy of our Lydia’s chromosome map. It is a very cool thing to have! Your Noah is adorable!!!!! 🙂 Thanks for your advocacy and for helping to educate!! 🙂

  3. Dear Noah’s Dad (and Mom),

    I want to thank you for the crucial information you’re putting out into the world to end the fear. Your son, like so many other sons and daughters, share their gifts so freely. I would love to interview you for an upcoming feature in our magazine, ACHIEVER, which highlights positive stories of regular people doing amazing things to make life more fulfilling and meaningful. If you would be at all interested, you can see a copy of the magazine at http://www.mylifelist.org/achievermag, and you can reach me at shelagh@mylifelist.org

    As a parent myself, I really appreciate your firsthand view of what “perfection” means. Hope to hear from you soon.


  4. Thanks for the quick overview, but since your wife is a doctor, maybe you could post some more details. My son is 2 1/2 and I’ve never been able to find a clear answer for: a) What information does the 23rd chromosome contain? (why are some things affected with DS like heart or lung conditions on some but not others, while other things are more consistent like facial features) and b) how does having an extra chromosome affect the characteristics of the person? Why doesn’t the body ignore the third copy? I just never understood why having too much a something can be a “bad” thing. IE (low muscle tone, smaller nasal passages, heart conditions, developmental delays etc.) I feel that I always see the same explanations about DS, but I don’t really understand much about the specific roles of chromosomes. I know if another chromosome was triplicated it would have different results with a different syndrome.

    • Lisa Hoffmann says

      I agree.. I love Noah and everything you are doing for him and all of us that are going thru the same having a child with down syndrome.
      I am also still confused with how some things are affected and not all. All I know is we were told our daughter has trisomy 21. She has very few characteristics, no medical conditions outside of low muscle tone and a few milestone delays, but nothing outside any other child’s possibility of development. I have read about the three types, but how do you know which you have?

  5. This is awesome, can’t wait to share it!

  6. Kara Ferris says

    Thank you so much for this very understandable description of Down Syndrome! I’m going to repost on iamviable!

  7. Cherokeevero says

    Amen, I have a wonderful son, Adam whom is Downs Syndrome he is 13 years old now, he is such a joy in our lives. He is translocation 21 and Adam has a younger brother 8 years old Vincent. I remember when the doctors wanted to test my husband and I to see whom carried the trait. We refused we understood that God doesn’t make mistakes. All children are a gift from God. And God chose us to care for His Special unconditional loving child and we are Blessed by Adam 🙂 Thank you fir taking the time to educate people and five comfort to new parents that our special children will to the things every other child does, but on their own time. God Bless Noah and his family.

    • blessedwithAlan says

      I have a son 14 with Downs Syndrome and like your Adam and all these blessed children he is the love of my life and the love of his 9 siblings. We learn so much thru him, the excitement of a battery(to work some toy) a toothbrush(cuz its just cool) a sibling he hasn’t seen in a few weeks(it has been forever). Everything they get is a joy, it is like Christmas 365 days a year. We can learn so much from these children that God gave to us. My oldest son said (I believe he was 14 and my youngest with Downs was just a a year old)..”maybe they are the perfect people God made and we are the ones that something went wrong with because they find nothing wrong with anyone in this world) I think my son is onto something here!

  8. Laurie Emanuele says

    Hi Rick – Love your website and positive karma. Today I came across your T21 explained page. I was surprised to see you have described T21 as a “chromosome abnormality”.

    I recommend the following definition, a quote by Rose Mordi, Nigeria at the UN conference on World Down Syndrome Day:

    “Down syndrome is a naturally occurring chromosome arrangement that has always been part of the human condition.”

    …..naturally occurring. Not an abnormality. Not a disorder. Not ‘What’s wrong with him?” Not a 90% abortion rate.

    I have a naturally occurring chromosome arrangement that has always been part of the human condition. My life has just as much value as any other, including yours.

    Can you imagine a doctor walking in and saying, “Your baby has Down syndrome. Don’t worry though. Children have been born with Down syndrome since the beginning of time. Down syndrome is a naturally occurring chromosome arrangement that has always been part of the human condition. Your child will be blessed to be a member of an amazing population of people that make this world a better place to live in.”

    Something as simple as the language we use could change the perception of how the world views my child and so many other children.

    What a wonderful, wonderful world it would be.

    Thanks for everything Rick!

  9. Cathy Frazier says

    I was recently in Florence, Italy and guess what I saw? The same billboard you have on your site. Everyone I was with loved it!

  10. Cute and sad at the same time

  11. This is amazing job Rick, since I come across with Noa’s Dad facebook page I was really happy knowing that me and family is not alone trying to give the best to our children with DS. My Saimon is now 2 yrs old and he gave us overwhelming love and joy each day of our lives. I have been following Noa’s video and fb update and he just adorable just like my Saimon. Thank you so much for sharing him with us. God bless you. (enclosed pic Saimon and granny)

  12. Dear good people there,
    Greetings for the day!!!

    My daughter who is 5months 10says got her tongue slightly geeeenish and extending it out more frequently since last 2days, will you please assist me with what all reasons could be behind such kinda activity or its just a play trick?

    Thanking you in anticipation.

    Kind regards,

  13. Chris Lucas says

    Noah’s Dad,
    I just stumbled onto your sight and something wonderful has happened. I have a niece who has had my great nephew, and he has Downs Syndrome. I was very scared until I started reading your sight. Tyler, my great nephew, is going on 3 and I have yet to find the courage to visit him. With these words of wisdom I believe that the strength has found me. My niece is in the middle of a move, but, I will make the effort to get to know Tyler as soon as they are settled in their new home. I now understand that the different a person is, the very unique that they become. I look forward to reading your posts and finally getting to know Tyler. I will not be afraid. I too have a brain injury that has disabled me the last 6 and a half years. There is no cure and the survival rate was 4 months and I am 49 years young. As you can see, I survived. I too am different and I hold no fear now…
    Thank You!!!

  14. Thank you for ALL you do Rick !!! I used this post this morning sharing DS awareness with a Co Worker.

  15. ali rehman says

    great i have got more help from this explination of down syndrom and feel so exited when found this post, it is explained in so easy way.
    Thanks to Noah’s DAD

  16. I used this for a research paper and it helped a lot! Thank you ?

  17. Great info that’s for sharing but you might want to check your grammar! It’s called Down’s Syndrome not Down ! The extra chromosome was discovered by a Dr Down therefore the syndrome is named after him as he discovered & named it therefore it’s Down’s Syndromme.

  18. Emily Price Carrigan says

    Love your blog and have been reading for quite some time. Just wanted to encourage you some more by offering that my daughter(age 20) is graduating in two weeks from The University of Central Missouri outside of St. Louis, Mo. This University is a typical four year U with Division 2 football, baseball, basketball etc. Our daughter graduated from high school in four years and wanted more education. At this school they have a program named THRIVE. Only 15 students are selected each year to attend. You have three interviews before you qualify,however, our daughter made it. This required her to fly by herself to and from Dallas to St. Louis, Mo. She loved every minute of her independence and
    living in a dorm atmosphere.
    Please do not put any limitations on your children that are born with Down Syndrome because you just never know how high they CAN and WILL fly 🙂

    Offering : HOPE all of the time,

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