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Extra or Missing Chromosomes

My nephew was born with an extra chromosome. He never developed past the stage of a two-month old, could only drink formula, and he lived until he was two years old. Is this genetic, and is there a name for it?

-A middle school teacher from Massachusetts

March 8, 2011

Your nephew had a genetic disorder called trisomy. That's just a fancy word for what you said: being born with an extra chromosome.

Trisomy is genetic, but it isn't often passed down from parent to child. In that way, trisomy is similar to many cancers. Both result from a random mistake.

A whole range of mistakes can happen in a normal cell and cause cancer. In the case of a trisomy, a very specific mistake happens when an egg or sperm gets made.

You have two copies of every chromosome. But an egg or sperm only has one. That one is chosen at random, through a process called meiosis.

When meiosis messes up, you can end up with too many or too few chromosomes. One extra chromosome in an egg or sperm means three in the fertilized egg and so trisomy.

Just like certain things increase your risk for cancer, trisomy has risk factors too. But they aren't really things that you can control.

Trisomies seem to be more likely if the mom is older. And certain groups are more prone to trisomy than others. But it's not like wearing sunscreen to prevent skin cancer.

And just like cancer, there are exceptions when the risk for trisomy gets passed down from parent to child. In cancer, there are certain genes that cause this. For trisomies, it's called a balanced translocation. About 1 in 1000 people has one. It means that a quarter of their egg or sperm cells will have an extra chromosome.

Balanced Translocations and Trisomy

Most of the time, chromosomes are units: they exist unattached to each other. That's how it should be.


An extra chromosome can be
passed on if it is attached
to another one.

Sometimes, though, part of one chromosome gets swapped with part of another, completely different, chromosome. That's called a translocation--when part of the genome moves to another place.

What if it's most of a chromosome? What if just about all of chromosome 22 and a small part of chromosome 9 switch places? This "balanced translocation" doesn't have much of an effect on the person. But it can definitely increase his or her chances of having a child with a trisomy.

These people aren't affected by the balanced translocation because nothing gets lost or repeated. Their genes are just in a different order now. But it does cause an increased risk for trisomy in their children.

Think back to meiosis. One copy of each chromosome gets chosen randomly. If one of your chromosome 9s actually has most of chromosome 22 on it as well, things can get hairy.

In the picture, you can see how the chromosomes get messed up. The child can end up with two copies of chromosome 22, one copy, a copy attached to chromosome nine, or no copies at all. So each child has an equal chance for trisomy, a missing chromosome (called monosomy), the same balanced translocation, or the usual assortment of chromosomes.

It's incredibly unlikely that you have to worry about a balanced translocation, though. They're very rare, and it would have shown up in the test that proved your nephew had an extra chromosome.

Too much of a good thing

Why should trisomy be so bad? Why do we need two, and only two, copies of every chromosome?

It comes down to balance. If your DNA is like a recipe for making you, then every gene is an ingredient. If you throw in too much of any ingredient, you can mess up the whole recipe. And there are lots of genes on a chromosome, so an extra chromosome means too much of a lot of different ingredients.

Of course, it all depends on which ingredients you're adding too much of. Since people have twenty-two chromosomes and two sex chromosomes, there are twenty-four different trisomies.

Some people are walking around right now with an extra chromosome and they don't even know it. Others are living with Down syndrome or, like your nephew, die soon after birth. Still others are never born at all: the extra chromosome causes a miscarriage.

Extra copies of the sex chromosomes usually don't do much. These are the chromosomes that determine whether you'll be a boy or a girl. Two X chromosomes means girl, and an X and a Y means boy.

People only use one X chromosome. Even women with three X chromosomes or men with two X chromosomes usually just use one copy. And the Y chromosome is a tiny piece of DNA with only a few genes. Most people don't even notice an extra X or Y chromosome, or even two or three. If you're making chocolate chip cookies, these would be like the chocolate chips: it's hard to add too many.

Other trisomies are more sensitive. Having three copies of chromosome 21 causes Down Syndrome. While that's unlikely to just go unnoticed, people with Down Syndrome can lead happy, fruitful lives. That's more like vanilla: there's some flexibility, but it changes the taste of the cookie.

These three cases are exceptions to the rule, though. Most trisomies result in miscarriage. Most of the infants that do survive to birth don't last more than a few weeks. In some ways your nephew was lucky. These chromosomes would be like baking soda. Too little or too much and you can mess up the recipe.

As you've experienced, trisomy can cause tragic disorders and death. It all comes down to a mistake that happens during meiosis.

Meiosis animation.


Elizabeth Finn


Sometimes a cell messes
up and a sperm or egg gets
an extra chromosome.
This is called nondisjunction.


Every trisomy is different.
This boy has three copies
of chromosome 21, giving
him Down syndrome.