Extra or Missing Chromosomes

My father is XYY. Is there something genetic that could have caused this when his father and mother conceived him? Or is this a totally random occurrence? Does him being XYY affect his offspring in anyway? If so how and why? Could it be possible it made him more likely to have girls?

-An undergraduate from California

January 15, 2009

Chances are, if you or anyone around you has XYY, you'd never know it. Most of these men don't look or behave any differently.

An XYY man is called this because he has an extra Y chromosome. Having an extra chromosome is usually fatal or can result in conditions like Down syndrome. But not with an extra Y.

XYY men are by and large perfectly normal. They tend to only find out they have a different number of chromosomes when they get a genetic test done.

Getting an extra chromosome is almost always a matter of random chance. It usually happens when something goes wrong when a sperm or egg is made. (In the case of XYY men, it's almost always something in their dad's sperm production that goes wrong.)

But XYY men themselves tend to have normal sperm. And so their children almost always have all the usual 46 chromosomes and are no more likely to be girls than are those of an XY dad.

The Extra Y Has Little or no Effect

Humans normally have 23 pairs of chromosomes, or 46 in total. The sex chromosomes make up the last pair.

The sex chromosomes are the ones that make you a male or a female. Men usually have an X and a Y chromosome and so are called XY. Women usually have two X chromosomes and so are XX. But some people have an X and two Y's, making them XYY.

Way back when, doctors thought that the extra Y would make men extra aggressive. In fact, for a while doctors thought that being XYY would make someone more likely to commit crimes! It used to be that people even called it Speck syndrome after the famous mass murderer Richard Speck.

But research has since proved that there's no link between violence and the extra Y. In fact, the vast majority of XYY men don't show any kind of symptoms and don't have any medical problems. They do tend to be a little taller than average, but only by an inch or two. Sometimes they can have some learning difficulties but that is also pretty uncommon.

The Extra Y Happens During Sperm Production

To explain how some men get an extra Y, let's talk a little bit about how we get chromosomes from our parents in the first place. I mentioned we have 23 pairs of chromosomes. For each pair, one chromosome comes from our mom and one from our dad.

But mom and dad have two copies of each of their chromosomes, too. So how do they just give one of each pair to their kids? By meiosis. Here's how it works (numbers correspond to the image on the right):

  1. Our chromosomes make a copy of themselves. Now there are 4 of each chromosome in the cell that will eventually go on to become an egg or sperm.
  2. This cell (with 4 copies of each chromosome) then divides once, making 2 cells. Since the chromosomes almost always get split evenly between the 2 cells, each cell now is back to 2 of each chromosome.
  3. These cells then divide again. The chromsomes again split between each of the resulting cells. So in the end, you usually get a sperm or egg with just one copy of each chromosome.

Sometimes something goes wrong in one of these steps of meiosis. Maybe at step 2, three of the chromosomes go into one cell and only one in the other. Or maybe at step 3, two copies go into one cell and no copies go into the other.

These sorts of mistakes are random and happen pretty frequently. Scientists call them nondisjunction events (click here to see what this looks like). If the mistake happens at step 2 it is nondisjunction in meiosis I. And if it happens at step 3, it is called nondisjunction in meiosis II.

Most of the time, these random mistakes end in miscarriage. This is because having an extra or missing chromosome is usually fatal. Some scientists think that up to 40% of fertilized eggs are miscarried because they are missing or have an extra copy of a chromosome.

But as I've said, there is almost no consequence from having an extra Y chromosome. So we see more cases of XYY than other cases of extra chromosomes because embryos with this extra Y can survive. In fact, scientists estimate that about 1 in 1000 men is XYY.

XYY Dads Tend to Have Equal Numbers of XX and XY Children

Remember, men have an X and a Y chromosome. So these guys usually make sperm that has either an X or a Y chromosome.

But what about an XYY male? From our earlier meiosis lesson, we might think that he would make four different kinds of sperm -- X, Y, XY, and YY.

We'd then predict four possibilities for his children, three of which would be male. The dad could have XX (female), XY (male), XXY (Kleinfelter's, another chromosome disorder), or XYY children. But that's not the case. XYY men generally don't have any more boys than XY men. And they don't have more cases of XYY or XXY children either.

Scientists aren't exactly sure why this is. They do know that the body has a lot of safeguards and checks to make sure that meiosis works properly. So many in fact that when XYY men make sperm, 99% of the time that extra Y is lost.

Of course scientists have theories about why this is. One of the more likely ones is that the body eliminates most sperm that have an extra or missing chromosome.

There may also be ways to make it less likely that an XYY egg is carried to term. There might be safeguards that make a woman's body more likely to miscarry an embryo with extra or missing chromosomes. The body wouldn't know that an XYY child is no different than an XY one.

As you can see, it's pretty amazing that the human body can, most of the time, figure out when there's just one extra chromosome hanging around. Scientists are working on figuring out how these protective mechanisms work, so they can understand and even prevent these random extrachromosomal disorders from happening!

Julia Oh, Stanford University

An extra Y (arrow) doesn't necessarily mean you'll have any problems.

XYY men have mostly normal sperm.