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# What is Down syndrome?

## What is Down syndrome?

The human body is an amazing thing. We really do come in all shapes and sizes. This is true for the things we can see (like height or hair color) and for the things we can’t see (like average heart rate). It seems like every time scientists try to find something that is truly the same for ALL people, they end up finding endless variations. Interestingly, these human differences can be found even at the level of our DNA! Some of these genetic variations are so small that they are unnoticeable, but some are substantial enough to change the way the body grows and develops.
• Down syndrome is a chromosomal (related to your DNA) disorder in which atypical cell division causes an extra portion of chromosome 21 to be present in some or all of a person’s cells.

## Where do our chromosomes come from?

Chromosomes are the tightly wound strands of genetic material that contain all of your DNA. Your chromosomes provide the instructions for how your body will develop before you’re born, and how it will function as you grow. Typically, babies are born with 23 pairs of chromosomes (46 chromosomes in total).
Diagram of a typical set human chromosomes
Half of your chromosomes come from your mother and half come from your father. Through a process called meiosis each of your parents creates a sex cell (either a sperm or an egg) that contains half of their chromosomes. When the sperm cell and the egg cell combine, they create a new fertilized cell (how cute!) that contains 46 chromosomes. That cell then starts making new copies of itself, through a process called mitosis, each with 46 chromosomes. (Check out this great video on mitosis and meiosis)
Diagram of mitosis and meiosis

## What happens with Down syndrome?

During both mitosis and meiosis, there is a phase where each chromosome pair in a cell is separated, so that each new cell can get a copy of every chromosome. However, sometimes during this process, a pair of chromosomes doesn’t separate evenly, which results in one of the new cells having an extra section of chromosome. This is called nondisjunction. With Down syndrome, various types of uneven chromosome separation result in a person having an extra copy (or partial copy) of chromosome 21.
There are three main types of Down syndrome: trisomy 21, mosaicism, and translocation.

### Trisomy 21

Trisomy 21 is the most common form of Down syndrome, accounting for about 95% of cases.start superscript, 1, end superscript This type of Down syndrome is caused by uneven separation of chromosome 21 during the creation of sex cells (this can happen in either the sperm or the egg cell), which leads to a fertilized egg with three copies of chromosome 21 instead of two. When the fertilized egg is developed, it passes along the extra copy of chromosome 21 to every cell in the body.
Diagram of genetic transmission of trisomy 21

### Mosaicism

The mosaic form of Down syndrome is much less common, accounting for about 1% of cases.start superscript, 1, end superscript In this form, the uneven separation of chromosome 21 happens shortly after an egg has been fertilized. The timing of this nondisjunction is important, because it leads to a person having some cells with the typical 46 chromosomes, and some cells with 47 (these cells have an extra copy of chromosome 21). Because only some cells have the extra chromosome, mosaic Down syndrome may have less prominent symptoms than trisomy 21.
Diagram of genetic transmission of mosaicism

### Translocation

In the remaining 4% of cases of Down syndrome, the extra genetic material is passed on to new cells in a slightly different way. Rather than failing to separate, translocation occurs when a portion of chromosome 21 breaks off during the replication process, and then attaches to another chromosome.start superscript, 1, end superscript So rather than getting a full extra chromosome, translocation results in cells with the typical 46 chromosome, plus a little extra chromosome 21. The genes contained in the extra portion of chromosome 21 can cause many of the symptoms of Down syndrome.

## What are the symptoms of Down syndrome?

Although the mechanisms by which a person receives the extra (or partial extra) 21st chromosome may differ between different forms of Down syndrome, the symptoms of the disorder are largely the same across types. The major symptoms of Down syndrome fall into three groups: physical development, intellectual disabilities, and other health problems.
Physical development: Down syndrome is associated with several distinct physical traits, including:
• Flattened facial features
• Smaller than average head and ears
• Protruding tongue
• Short and broad neck, hands, and feet
• Poor muscle tone
Babies with Down syndrome are typically of average size at birth, but as they age, are much slower to grow than children without the disorder. As a result of poor muscle tone, delays in sitting up, crawling, and walking are also common.
Intellectual Disability: It is common for people with Down syndrome to have mild –moderate intellectual disabilities. Delayed development of language and speech, poor attention and impulse control, and learning problems are common characteristics.
Other health problems: People with Down syndrome have been found to be at much higher risk for developing many other health problems, including:
• Hearing loss/impairment
• Congenital heart disease
• Vision problems
• Epilepsy
• Digestion problems
• Mental health and emotional problems
All of the symptoms of Down syndrome exist on a continuum, meaning that they can range from minor to severe. This also means that the same person could have some severe symptoms and some very minor symptoms. (Be careful not to assume a person has all of the symptoms of Down syndrome just because they have some of them!)

## What causes Down syndrome?

Although we understand the how of Down syndrome, we still don’t understand the why very well. In other words, scientists have figured out how people end up with an extra 21st chromosome (see above), but it remains unclear exactly why those chromosome-separating problems happen. Research suggests that in most cases, the chromosome copying error happens at random, and is not caused by any harmful behavior by the parents before or during pregnancy.cubed Risk factors for Down syndrome include increased age of mother during pregnancy (35 or older) and already having a child with Down syndrome.

## How common is Down syndrome?

Down syndrome is the most common chromosomal disorder, and it affects people from all racial, cultural, and economic groups.start superscript, 1, end superscript Worldwide, it is estimated that approximately 1 in every 1,000 people are born with Down syndrome (that’s about 3,000-5,000 people every year).start superscript, 4, end superscript

## Can you prevent Down syndrome?

Due to a lack of understanding of the cause of Down syndrome, it is unclear exactly how to prevent it. Because some people are at higher risk for having a child with Down syndrome, it can be helpful to meet with a genetic counselor before becoming pregnant. Genetic counselors can help you understand the probability of your child having Down syndrome.
Prenatal testing for Down syndrome is available. This means that during pregnancy (usually the first or second trimester) various tests can be performed to diagnose a fetus with Down syndrome. Some parents decide to terminate a pregnancy when Down syndrome is detected, to prevent a child from being born with the condition.

## How is Down syndrome treated?

Because every person with Down syndrome has specific strengths and needs, there is no best way to treat the disorder. Typically, children with Down syndrome will work with a treatment team made up of doctors, psychologists, and therapists that specialize in the areas of that child’s symptoms. Early learning therapies are commonly used to help with the learning and language problems associated with Down syndrome. Physical therapy can help build muscle control and movement coordination.

## Consider the following

Down syndrome is a DNA disorder, so it must be inherited, right? In fact, Down syndrome is very rarely inherited directly from a person’s parents. Why might that be? Approximately 96% of cases of Down syndrome are caused by nondisjunction in either the sex cells of the parents, or the fertilized egg (trisomy 21 or mosaicism). In either case, the failure of chromosome 21 to separate is not caused by the parents having Down syndrome (meaning it is not inherited). A parent with the translocation form of Down syndrome could pass along the disorder to their child, but it is estimated that this only occurs in approximately 1% of cases.

## Want to join the conversation?

• Are their any other ways you can get down syndrome besides what they have told us.
• No, down syndrome is a genetic disease. Persons affected have an extra copy of chromosome 21. This can only happen at conception. The syndrome is apearant at birth, so older children or adults cannot randomly get it, when they dont already have it.
• I'd like to know what are the differences in symptomatology related to the timing of the mosaicism(fifth paragraph, last sentence)?

kind regards,

Wijnand van den Berg
• In Down Syndrome, which gender is most likely affected on, females or males?
• How can a failure in the oocyte's meiosis II lead to mosaicism? Wouldn't it just be a complete trisomy 21? Since meiosis II happened after fertilization but before the zygote's first division, wouldn't all cells have trisomy 21?
• With mosaicism it says that the incorrect separation happens shortly after fertilisation of the egg cell. Though in the drawing there's also a separation at the first meiosis. Is this drawn incorrectly or does the explanation miss something? Because right now the numbers don't add up (24 from egg cell + 23 from sperm cell should give 47 like with trisomy 21, but it shows 46...)
• You are correct. Good eye. This needs to be changed. I am sorry, I was wrong.
• In the diagrams for Trisomy 21 and mosaicism, the fertilized eggs undergo meiosis II. Why? Is this a mistake or am I just not understanding it?
(1 vote)
• Egg cells that are ready to be fertilized are temporarily stuck meiosis I, and won't go into meiosis II until fertilized (which is different from sperm cells!). If you remember meiosis should produce 4 cells (and it does in males), but that seems a bit strange because you also know that in humans normally only one egg cell is is ovulated right?

It turns out in meiosis I the one cell becomes the egg and the second becomes a very small Polar Body on the side of the egg. It then waits until a sperm fuses with it membrane, and at that moment the oocyte undergoes meiosis II, creating another polar body and becomes the fully mature "Ovum" which doesn't last long because the sperm 'pronucleus' is already there and headed to join with the ovum's 'pronucleus' to create a "Zygote" with its nucleus.
• It is stated that down syndrome is not inherited, then why is it also stated that one of the risk factors can be already having a child having down syndrome. Isn't is purely by chance?
(1 vote)
• Typically, Down syndrome is by incorrect chromosomal separation during meiosis. If the person did it once, then they have a higher likelihood of doing it again, that is what is observed. It is also observed that older women have higher incidence. We are unsure why this happens but we know it is not simple chance. https://www.nichd.nih.gov/health/topics/down/conditioninfo/Pages/risk.aspx#f3