Chimeras, Mosaics, and Other Fun Stuff
I recently read about a case where a person had both a pair of X chromosomes and an X and a Y chromosome. How does that happen?
-A curious adult from California November 12, 2009 What you're probably talking about are people who have two sets of cells in their body. One set has an X and a Y chromosome. These cells are male. The second set of cells has two X chromosomes and so is female. Whether this has any effect on a person depends on the percentage of each kind of cell the person has. If he has mostly XY cells, then he might not even know about his XX cells. Same thing if she has mostly XX cells. There can definitely be issues with someone who has a more even split of XX and XY cells. These folks are often true hermaphrodites -- they have both male and female qualities. This sort of thing isn't restricted to just the X and the Y chromosomes either. It can happen with any of the other chromosomes too. In these cases, some cells might have an extra chromosome or be missing one entirely. Again the effects depend on the percentage of cells that have the extra or missing chromosome. And on which chromosome is affected. A case in point is Down syndrome. Down syndrome happens when someone has an extra chromosome 21. In around 1-2% of Down cases, only some of the person's cells have the extra chromosome (this is called mosaic Down syndrome). How severe these people's symptoms are depends on the percentage of cells that have the extra chromosome. For now, though, we'll focus on cases where people have both XX and XY cells. There are a couple of ways this can happen. In the first way, an XX twin and an XY twin fuse very early on in development. The end result is a person with both sets of cells. This is called chimerism. In the second way, a fetus starts out with an extra X chromosome. Early in development, something happens when the egg divides so that each new cell gets an uneven number of chromosomes. As the cells continue to divide, you end up with someone with two sets of chromosomes. This is called mosaicism. What I'll do next is quickly go over each of these possibilities in more detail. I'll provide some figures to try to make it easier to understand. Chimeras As I said, chimeras happen when fraternal twins fuse very early on. At first this strikes many people as a bit odd. Our bodies won't tolerate most organ transplants so why do they accept these cells? The simple answer is that the fusion happens before the immune system develops. Once it does, it sees both sets of cells as part of the body it needs to protect. Chimeras are thought to be rare and most people don't know they are one until they have a genetic test done. Then there can be problems with organ transplants. Or even criminal investigations. The exception to this is XX/XY chimeras. These folks often have characteristics of both males and females and so start out with issues that other chimeras do not. You can get a feel for how this can happen by looking at the figure to the right. As you can see, a set of boy-girl twins fuses early on to create a single person. A brother and sister rolled up into one fetus. After the fusion, things progress normally. Each set of cells keeps dividing until you get to the trillions of cells found in an infant. In this case, because there is an equal number of XX and XY cells, the chances are high that the child will turn out to be a true hermaphrodite. Mosaic Mosaics are probably more common than chimeras but a bit more difficult to explain. Part of the problem in explaining them is that there are so many different ways it can happen. Luckily for us, there are many fewer ways to end up an XX/XY mosaic. In the image on the left, I show one way it might happen. As you'll see, it takes two mistakes in cell division. We start off with a Y sperm. It goes on to fertilize an egg that has two X chromosomes. (It is also possible that the sperm has an XY and that the egg has a single X.) An egg or a sperm with an extra chromosome isn't that uncommon. The reason we don't see more cases of missing or extra chromosomes is that most of them result in early miscarriage. The X and Y chromosomes are two of the few where extra or missing copies are tolerated. Now we have a fertilized egg with two X chromosomes and a Y chromosome. If nothing else happens, this fertilized egg would go on to develop into a male with Klinefelter syndrome. But in this case, a second mistake happens. When the egg divides into two cells, one cell gets an X and a Y chromosome while the other gets two X's. After this mistake, things progress normally. Each set of cells keeps dividing until, like in the chimera case, a true hermaphrodite may be born. Varying Percentages In both of these examples, the person ends up with half XX cells and half XY cells. But it doesn't have to be that way. If the chimera fusion happens when the two embryos are different sizes, you can end up with different percentages. For example, if the XX twin was at an 8 cell stage and the XY twin was a single cell when they fused, then the person might have 1/9 XY and 8/9 XX cells. It can also be that more of one set of cells is eliminated during development. Lots of cells die before a baby is born (the cells in the webs between our fingers are a good example). It may be that more of one set of cells happened to die off. For the mosaic, it is also possible for someone to have three sets of cells -- XXY, XX, and XY. This would happen if the XXY fetus divided a few times and then one of its cells underwent the mistake that caused one cell to be XX and the other to be XY. As you can see, there are lots of ways to end up with two sets of cells in your body. Most are pretty rare though. Which is why you don't hear about this sort of thing very often.

Some people have two sets
of cells. One set is XY and
the other XX.

Chimeras, Mosaics, and Other Fun Stuff