What Is Down Syndrome?

Your genes hold the keys to how your body looks and works. They’re behind everything from your hair color to how you digest your food. So if something goes wrong with them, it can have far-reaching effects.

People with Down syndrome are born with an extra chromosome. Chromosomes are bundles of genes, and your body relies on having just the right number of them. With Down syndrome, this extra chromosome leads to a range of issues that affect you both mentally and physically.

Down syndrome is a lifelong condition. Although it can’t be cured, doctors know more about it now than ever. If your child has it, getting the right care early on can make a big difference in helping him live a full and meaningful life. 

Effects of Down Syndrome

Down syndrome can have many effects, and it’s very different for each person. Some will grow up to live almost entirely on their own, while others will need more help taking care of themselves.

People with Down syndrome tend to have certain physical features in common. For example, they often have flat noses and small ears.

Their mental abilities will vary, but most have mild to moderate issues with thinking, reasoning, and understanding. They’ll learn and pick up new skills their whole lives, but may take longer to reach important goals like walking, talking, and developing social skills.

Many people with Down syndrome don’t have any other health issues, but some do. Common conditions include heart problems and trouble hearing and seeing.

Causes

Normally, each cell in your body has 23 pairs of chromosomes. One chromosome in each pair comes from your mother. The other comes from your father.

But with Down syndrome, something goes wrong and you get an extra copy of chromosome 21. That means you have three copies instead of two, which leads to the signs and symptoms of Down syndrome. Doctors aren’t sure why this happens. There’s no link to anything in the environment or anything the parents did or didn’t do.

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While doctors don’t know what causes it, they do know that women 35 and older have a higher chance of having a baby with Down syndrome. If you’ve already had a child with Down syndrome, you’re more likely to have another one who has it as well.

It’s not common, but it is possible to pass Down syndrome from parent to child. Sometimes, a parent has what experts call “translocated” genes. That means some of their genes aren’t in their normal place, perhaps on a different chromosome from where they’d usually be found.

The parent doesn’t have Down syndrome because they have the right number of genes, but their child may have what’s called “translocation Down syndrome.” Not everyone with translocation Down syndrome gets it from their parents -- it may also happen by chance.

Types

There are three types of Down syndrome:

  • Trisomy 21. This is by far the most common type, where every cell in the body has three copies of chromosome 21 instead of two.
  • Translocation Down syndrome. In this type, each cell has part of an extra chromosome 21, or an entirely extra one. But it’s attached to another chromosome instead of being on its own.
  • Mosaic Down syndrome. This is the rarest type, where only some cells have an extra chromosome 21.

You can’t tell what type of Down syndrome someone has just by how they look. The effects of all three types are very similar, but someone with mosaic Down syndrome may not have as many signs and symptoms because fewer cells have the extra chromosome.

WebMD Medical Reference Reviewed by Roy Benaroch, MD on March 18, 2017

Sources

SOURCES:

National Institutes of Health: “The Genetic Basics: What Are Genes and What Do They Do?,” “Learning about Down Syndrome.”

KidsHealth: “Down Syndrome.”

Mayo Clinic: “Down Syndrome.”

National Health Service (U.K.): “Down’s Syndrome.”

CDC: “Facts about Down Syndrome.”

March of Dimes: “Down Syndrome.”

MassGeneral Hospital for Children: “Translocation Down Syndrome.”

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