-A middle school student from California
February 5, 2009
Wow, there aren't just two-headed snakes, but two-headed sheep, pigs, cats, dogs, fish, and even people! With two separate heads (and two separate brains), how can they survive? Wouldn't the two heads fight with each other? And how would they control their body?
Well, they do have a lot of problems and most of them don't survive very well in the wild. They can't decide which direction to go and sometimes they even try to attack and swallow the other head!
Two-headedness is really a severe example of conjoined twins. These twins happen when identical twins don't separate completely. Instead of two twins, you end up with twins who are still attached.
The attachment can be as little joining as just a piece of skin or cartilage connecting the two twins. Or in more severe cases, they can share a whole body but separate heads!
Ending up with Two Heads
Identical twins happen when a single egg is fertilized but then splits into two separate embryos. Each embryo then goes on to grow into a separate person. Depending on when the split happens, twins can either grow in their own sac with their own placenta or they can share a sac and placenta.
Conjoined twins can happen in this second case. But it isn't common"¦in humans it only happens in around 1 in 50,000 to 200,000 pregnancies.
There are two theories for how conjoined twins come about. One is that they happen when the fertilized egg splits incompletely. Another is that the embryo split is complete but for some reason the two new embryos re-fuse later on.
Twins can end up fused at various places. Where the twins end up attached depends on a lot of factors, like how they were oriented in the womb, the time and location of splitting, etc.
Sometimes, one of the twins can end up more dominant than the other. In this case, one of the twins can die and be absorbed by the other twin. If the absorption is complete, one of the twins vanishes. But, if the absorption is just partial, the remaining twin can have extra arms, legs, or even heads that are leftover from that other fetus!
Having conjoined twins doesn't usually run in families. This is because identical twinning doesn't run in families
either. It is a random event.
But some cases of having extra limbs can be genetic. One of the best understood is polydactyly, which is when people have extra fingers and toes.
Genetic Causes of Duplication
There are lots of genetic ways to end up with extra fingers and toes. Here I'll focus on just one case.
Most cases of polydactyly can be accounted for by changes in just one of a few different genes. This is surprising because making fingers is such a complicated task. Lots of different genes all need to work together to make 10 fingers and 10 toes.
So how can just one change in a gene cause something like an extra finger? Because this one gene can affect lots of other genes, too.
Recall that genes are our basic instruction manual for building and running all the things in our body. These genes are "read" by cells and made into proteins. Each protein then goes on to do a specific job in the cell.
Some proteins like hemoglobin carry our oxygen. And others like insulin help us use the food we eat.
It's very important that the right proteins get made and in the right amount. If we don't have enough hemoglobin, we have trouble breathing. Or if we don't have enough insulin, we get the disease diabetes.
That's why cells have lots of ways to control when, where and how often a gene is read. One of the ways cells do this is by using proteins called transcription factors* (TFs).
TFs stick to a gene and control how often the gene is read. Lots of copies of each TF are made and they usually stick to and control a bunch of genes all at once.
Some TFs called activators make cells make extra proteins. And some TFs called repressors cause cells to make less protein.
Well, one reason polydactyly happens is because of changes in a single gene, Gli3. GLI3 is one of these repressor TFs.
One of its jobs is to keep two genes, dHand and Sonic Hedgehog (Shh) from being read too often. When GLI3 isn't doing its job right, too much dHAND and SHh proteins get made. And too many fingers and/or toes get made, too.
Some people with polydactyly have mutations in GLI3 that cause it either to not work at all or that cause it to work only very weakly. So basically, the embryo ends up with too much SHh and dHAND, causing the extra fingers and toes.
This kind of polydactyly can be inherited from your parents. It is an autosomal dominant trait.
Autosomal means that boys and girls are just as likely to get it (it isn't sex-linked). Dominant means that you only need to have one copy of the polydactyly version to end up with extra fingers and/or toes. In fact, if you have one copy, you will have extra digits!
So if just one parent has this mutation in Gli3, their kids have a 50% chance of getting it too. If both parents have it, their kids have a 75% chance of having extra fingers or toes!
Polycephaly (having multiple heads or faces), or polymelia (having extra limbs) can also have some genetic causes. But, these are usually associated with other serious genetic defects (it's not well understood for either of these cases), or more frequently, are the result of twinning or another problem during development.
* These proteins are called this because the act of reading a gene is called transcription.