Where is the key to curing cancer?

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If you put five people in the same room, the chance of any two of those five having the same name is pretty low. However, let's make that 150 people. You just might find yourself with three Graces and a handful of Davids.

Now, think about how many genes we have in our bodies. In case you're curious, that count is between 20,000 and 25,000 [source: U.S. National Library of Medicine]. That's a lot of genes, which means there are a lot of opportunities to see the same thing crop up repeatedly -- like mistakes.

Things go wrong frequently with our genes, and when they do, it's called a mutation. Mutations change our genetic code. Our genes hold the directions for making the proteins in our body, and without the right ones, much-needed proteins can't be made. Therefore, a mutation that messes up the directions for an important protein can lead to health issues.

Most of the time, our genes are able to fix themselves. The problem is when the error can't be rectified and the mutation results in a disease. Does this mean that every genetic mutation that sticks is tied to a disease?

Certainly, a lot of genetic issues are partnered with diseases. However, it's more complicated than pointing fingers at our genes, especially since mutations are the culprit. Some diseases have a direct origin from a genetic mutation, but others are more complex -- so complex that there are several ways genetic diseases happen. Let's look at four different ways genetic diseases arise.

The first type of genetic disease we'll explore is the rare mitochondrial disease. Mitochondria are in charge of feeding our cells. These master chefs take the energy from our food and turn it into something our picky cells are willing to use. Mitochondria may not be in the center of the cell where most of our genetic action takes place, but they do have a little DNA of their own [source: U.S. National Library of Medicine]. Diseases brought on by genetic mutations in our mitochondria are referred to, sensibly, as mitochondrial diseases.

Next up are chromosomal diseases. The DNA in the center of our cells links up in pairs to form our chromosomes. We get 23 chromosomes from our mom and 23 from our dad, for a total of 46. A chromosomal disease can occur when the sequence of chromosomes isn't complete or is broken. An example of a chromosomal disease is Down syndrome. Individuals with this syndrome have an extra copy of their 21st chromosome, which is why it's sometimes called Trisomy 21.

Advance to the next page for the final two -- and a look at the future.