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Chromosomes

My daughter was told that her baby had two chromosomes split and rejoin with the other's half. In other words chromosomes A and B split half of A joined with half of B and the other two halves the same. What does this mean as far as the baby's health and development?

-A curious adult from California

February 26, 2009

Chromosome swaps like this are surprisingly common. Scientists call them "translocations". About 3 out of every 1000 babies are born with one.

And they usually aren't a big deal. There is almost never any obvious sign of something wrong with these people. Until they try to have kids, that is.

Chromosome swaps make it more likely to have a miscarriage. So, people with these swaps sometimes find it harder to have kids.

Chromosomes Store the Instructions for Making You

You may recall that chromosomes store the instructions for making you. A large part of these instructions are found in your genes, which are scattered throughout your chromosomes.

Think about your chromosomes like the drawers of a filing cabinet. The filing cabinet has files of instructions for making you. Each file is a different gene.

To make you, we need every file to be there. So we need to be careful not to lose too many files.

But moving files from one drawer to another isn't a big deal. All the information is still there.

A chromosome swap is just like moving these files around. All the instructions for making you are still there-they're just in a different place.

Incomplete Information Can Lead to Miscarriages

Let's explain why your grandchild may be at an increased risk for miscarriages in the future. Real life is a bit more complicated than filing cabinets. But I think I can still use the analogy.

We all have two copies of each of our chromosomes, one from mom and one from dad*. In our example, it is like we each have duplicate filing cabinets. Each filing cabinet has drawers that have the same information.

Imagine that for whatever reason, we want to fill up a new cabinet with drawers from both of these filing cabinets. So we take drawer one from the right filing cabinet, drawers two and three from the left, etc.

The end result is a new cabinet with all the information that was in each of the starting cabinets. This is how it works when a sperm or egg gets made.

Instead of drawers, one of each duplicate chromosome is put into an egg or sperm. Then, when the sperm fertilizes the egg, you end up with two sets of chromosomes again. It's like moving one filing cabinet each from mom and dad into the same office.

Remember that with a translocation, some files got moved between drawers. Let's say some of the files from drawer one of the filing cabinet on the right are moved to drawer one of the cabinet on the left.

Together, we have two full sets of instructions. But now let's make an egg or a sperm.

Again, we fill a new cabinet with drawers from the right and left cabinet. So we take drawers one and two from the right filing cabinet, drawer three from the left, etc.

Now when we combine this filing cabinet with one that has a complete set of instructions, we don't have two copies of every file. One of our filing cabinets is missing files because drawer one is from the filing cabinet on the right.

This is what happens with translocations too. Instead of missing files, we have missing genes. And if a fetus is missing too many genes, it is often lost as a miscarriage.

OK, so that's filing cabinets. Let me give you an example that deals with real chromosomes. To simplify, we'll deal with two pairs of chromosomes instead of the usual 23.

Translocations Without a Filing Cabinet

Let's just look at 2 chromosomes. So a typical person has two copies of each of these chromosomes. We'll represent them this way:

Imagine that someone has a translocation where part of these two chromosomes have swapped. They then have the following chromosome pairs:

Now this person decides to have kids with someone who has a normal pairing of chromosomes. Remember, when a sperm or egg is made, only one from each pair of chromosomes goes in. These are the possible sperm and eggs from these two parents:

Now here are the possible combinations when the sperm fertilizes the egg:

Notice the two on the bottom (C and D) are missing some information and have extra of other information. For example, C has extra information from chromosome 2 and not enough from chromosome 1. D is just the opposite. Most likely they will not go to term.

Possible Problems with Translocations

Sometimes a chromsome swap isn't as clean as we have described. Occasionally a swap happens in the middle of a gene. Then there can be problems.

Think about our filing cabinet again. Imagine that as we're moving files from one drawer to another, we leave part of a file behind. Now the incomplete file may not be useful anymore.

The same thing is true with genes and translocations. Sometimes the swap makes a gene more than useless. It can make it dangerous.

Many cancers, for example, can happen when a swap happens in the middle of an important gene. This can be a gene that is needed in the body, but only in small amounts. A swap can sometimes cause the body to make too much of the gene.

But as I said, thee kinds of things don't happen a lot. Usually there is no sign that there is one of these translocations until the person tries to have kids.

*The exception is the X and the Y chromosomes in men.


Chris Franco


Our pair of chromosomes
is like having a pair
of filing cabinets.


Chromosome rearrangements like this
are surprisingly common.