Chimeras, Mosaics, and Other Fun Stuff

Why can't mules breed? I understand that a horse and a donkey make a mule but why can't 2 mules have a baby mule?

-A middle school student from Michigan

June 20, 2007

You're right, a horse and a donkey can have kids. A male horse and a female donkey have a hinny. A female horse and a male donkey have a mule.

But hinnies and mules can't have babies of their own. They are sterile because they can't make sperm or eggs.

They have trouble making sperm or eggs because their chromosomes don't match up well. And, to a lesser extent, because of their chromosome number.

A mule gets 32 horse chromosomes from mom and 31 donkey chromosomes from dad for a total of 63 chromosomes. (A horse has 64 chromosomes and a donkey has 62).

To understand why this is a problem, we need to understand how sperm and eggs are made. And to understand that, we need to go into a bit more detail about chromosomes.

Remember, we have two copies of each of our chromosomes -- one from mom and one from dad. This means we have two copies of chromosome 1, two copies of chromosome 2, etc. However, this isn't entirely true for the mules.

The mule has a set of horse chromosomes from its mom. And a set of donkey ones from its dad.

These chromosomes aren't really matched sets like in a horse, a donkey, or a person. In these cases, a chromosome 1 is very similar to another chromosome 1. It looks pretty much the same and has nearly the same set of A's, G's, T's and C's. For example, two human chromosome 1's differ only every 1000 letters or so.

But a donkey chromosome doesn't necessarily look like a horse one. And the poor mule even has an unmatched horse chromosome just sitting there.

To make a sperm or an egg, cells need to do something called meiosis. The idea behind meiosis is to get one copy of each chromosome into the sperm or egg.

For example, let's focus on chromosome 1. Like I said, we have one from mom and one from dad. At the end of meiosis, the sperm or egg has either mom's or dad's chromosome 1. Not both.

This process requires two things. First, the chromosomes have to look pretty similar, meaning they are about the same size and have the same information. This will have to do with how well they match up during meiosis.

And second, at a later critical stage, there has to be four of each kind of chromosome. Neither of these can happen completely with a mule.

Let's take a closer look at meiosis to see why this is. The first step in meiosis is that all of the chromosomes make copies of themselves. No problem here...a mule cell can pull this off just fine.

So now we have a cell with 63 doubled chromosomes. It is the next step that causes the real problem.

In the next step, all the same chromosomes need to match up in a very particular way. So, the four chromosome 1's all need to line up together. But this can't happen in a mule very well.

Like I said, a donkey and a horse chromosome aren't necessarily similar enough to match up. Add to this the unmatched chromosome and you have a real problem. The chromosomes can't find their partners and this causes the sperm and eggs not to get made.

So this is a big reason for a mule being sterile. But how is the silly thing alive at all?

Well, there are a couple of reasons. First, having an odd number of chromosomes doesn't matter for every day life. A mule's cells can divide and make new cells just fine. Which is important considering a mule went from 1 cell to trillions of them!

Chromosomes sort differently in regular cells than they do in sperm and eggs. Regular cells (called somatic cells) use a process called mitosis.

Mitosis is like the first step of meiosis. The chromosomes all make copies of themselves. But instead of matching up, they just sort into two new cells. So for the mule, each cell ends up with 63 chromosomes. No matching needs to happen. And our lone horse chromosome is fine.

The other reason a mule is alive is that nothing on the extra or missing chromosome causes it any harm. This seems obvious at first except that usually having extra DNA causes severe problems. In people, extra chromosomes usually result in miscarriages. Sometimes though, a child can survive with an extra chromosome.

For example, people with an extra chromosome 21 have Down syndrome. Having all of the extra genes on that extra copy of chromosome 21 cause the symptoms associated with Down syndrome.

So having extra chromosomes often leads to real problems. But the mule is by and large OK.

The extra genes must not be that big a deal for the mule. In other words, the extra genes on the horse chromosome do not cause problems for the every day life of a mule.

So mules are sterile because horse and donkey chromosomes are just too different. But they are alive because horse and donkey chromosomes are similar enough to mate.


Monica Rodriguez


A mule can not make
sperm or eggs.


Horse and donkey chromosomes
are too different to let mules
breed successfully.


Extra chromosomes usually lead to problems.
Here we show the chromosomes of someone
with Down syndrome.
Note the extra chromosome 21.