Extra or Missing Chromosomes

My son was born with full trisomy 13. Is it possible to say how early the chromosome split the wrong way? In other words, was it before conception or very soon after? Or is it impossible to know? Thank you.

-A curious adult from Oregon

January 22, 2009

I am very sorry to hear about your son. Trisomy 13, also known as Patau syndrome, is a very severe condition. It affects roughly 1 in 10,000 births.

Patau's happens when someone gets an extra whole, or partial, chromosome 13. Since you have said that your son has a full trisomy, we'll focus on the situation where someone gets an entire extra chromosome.

As your question suggests, someone can end up with an extra chromosome because of something that happens before or after an egg gets fertilized. A clue to when it happened can be found in how many cells have the extra chromosome.

If all of a patient's cells have the extra chromosome, then the trisomy probably occurred while the egg or sperm was being made. If only some of the cells have it, then it probably happened after conception.

To understand why this is, we need to first understand how sperm and eggs get their chromosomes. We also need to explore how cells divide.

Getting an Extra Chromosome Before Conception

You might remember that humans have 23 pairs of chromosomes. We get one set of 23 from mom and the other set from dad.

To make this work, an egg gets half of mom's chromosomes and the sperm gets half of dad's. At fertilization, one of mom's eggs joins with one of dad's sperm. If everything works out OK, then the fertilized egg has 23 pairs of chromosomes -- one set from mom and one set from dad.

Eggs and sperm get half of the normal number of chromosomes through a process called meiosis. Let's look more closely at this process to understand how an egg or sperm might get an extra chromosome.

To make it simple, let's only think about one pair of chromosomes. Here's how meiosis works:

  1. A "mother" cell duplicates each pair of chromsomes so now there are 2 of each (4 total).

  2. The cell then divides and each "daughter" gets one of the duplicated pairs (each has 2).

  3. These cells then divide again, and each gets one of the originals.

The end result is a cell with a single chromosome.

This process is very complicated so it isn't surprising that cells sometimes get it wrong. Occasionally the separation of the chromosomes doesn't happen correctly and the pairs get stuck together. This means that an egg or sperm gets an extra copy of a chromosome. This might happen at either the first or second division.

Suppose this happens in one of the mother's eggs. When that egg is fertilized by one of dad's sperm, there are now three copies when there should have only been two.

When this happens, usually every cell gets an extra chromosome. But sometimes people end up with some cells that have the extra chromosome and some that don't. This situation usually means that the mistake happened after conception.

Getting an Extra Chromosome After Conception

At the end of the last section, we had a fertilized egg. All people start out as this single cell and then go on to become trillions of cells.

To get there, each cell needs to divide many times. And each time a cell divides, its chromosomes needs to be copied and distributed correctly. This process is called mitosis.

Mitosis is really just like the first step of meiosis. A cell first copies its chromosomes. It then divides and each cell gets a complete set of chromosomes. Usually.

Sometimes, again, the chromosomes get stuck together. Now you can end up with one cell that has an extra chromosome and one cell that is missing a chromosome.

When the mistake happens determines what percentages of cells have missing or extra chromosomes. For example, if the mistake happens when the first cell divides, then 50% of the cells will have an extra chromosome and 50% will be missing one. If the mistake happens in one cell at the 100 cell stage, then only 1% of cells will have an extra or missing chromosome.

The bottom line is that if every cell has an extra chromosome, then the mistake probably happened before conception. If only some have the extra chromosome, then it probably happened after conception.

Could we tell if the trisomy happened in mom or dad?

You might be curious if we can tell whether the trisomy happened in the mother or father. Scientists can learn this by performing further tests, but the tests are very laborious. And they are not particularly useful to the parents.

Ending up with an extra chromosome is almost never mom or dad's fault. It is a natural mistake that the body sometimes makes in a very complicated process. So, for the parents, it doesn't really matter who the extra chromosome came from.

However, scientists do want to figure this out to better understand how meiosis works. And whether a certain meiotic mistake is more common in conditions like Down or Patau syndrome.

Remember that everyone's chromosomes are a little bit different. Because of these differences, scientists are able determine from which parent the extra chromosome came.

They have done just this for Down syndrome. In Down syndrome, people have an extra copy of chromosome 21. Scientists have determined that 90% of the time the extra chromosome happens in the mother's egg, 8% in the father's sperm, and 2% happen after conception.

It is likely that the numbers for Patau Syndrome are similar. We don't really know why it occurs this way. We do know that it happens more often with increasing age of the mother. (Find out more here).

Again, I am so sorry to hear about your son. I hope this article helps you understand a bit more about how these syndromes occur.


Jared Wenger



Extra chromsomes can happen before
or after conception.
Karyotype of Down syndrome.


Scientists can tell whether
an extra chromsome came
from mom or dad.