The idea that we are all 99.9% the same came from looking for differences in our DNA letter by letter. We see the newly discovered differences by looking at big chunks of our DNA all at once.
Let's use a cookbook analogy to show you what I am talking about here. Our DNA has 6 billion letters cut up into 46 different strings of "text" (called chromosomes). Imagine that this DNA is a library of 46 cookbooks.
Just like cookbooks, our chromosomes have recipes too. The recipes are made of sets of instructions called genes. And the genes are written in three letter words called codons.
Until recently, most scientists focused on small changes within a gene. For example, imagine this set of instructions:
Get a glass. Fill the glass with milk. Add 2 tbsp of chocolate powder. Stir.
Here are a couple of single letter differences that completely change the meaning of this recipe:
Get a glass. Fill the glass with silk. Add 2 tbsp of chocolate powder. Stir.
Get a glass. Fill the glass with milk. Add 2 tsp of chocolate powder. Stir.
Now instead of a glass of chocolate milk, you get a glass of chocolate silk. Or in the second case, a very weak glass of chocolate milk.
This is how DNA works too. A change in a letter can cause a change in how a gene gets used. Or whether it gets used at all.
The newly discovered changes, called copy number variants (CNVs), are different than this. They are more like repeats of the same instructions. For example, imagine the recipe now says:
Get a glass. Fill the glass with milk. Add 2 tbsp of chocolate powder. Add 2 tbsp of chocolate powder. Stir.
Now you're going to get a stronger chocolate milk. Or imagine the whole recipe just repeats. Now you get twice as much chocolate milk.
Until a few years ago, scientists thought that most of our differences came from small changes in our DNA. The idea was that every 1000 letters or so, you and I have a different letter. These 6 million differences made me distinct from you.
But over the past few years, scientists have started to notice changes where big chunks of DNA are repeated. Or missing.
As they looked harder, more and more of these changes became apparent. As of November 2006, more than 600 of these CNVs had been identified that covered 104 million DNA letters (called bases). That's 4% of our DNA!
Now a new paper in the journal Nature shows there are even more than this. The researchers found 1447 of these big changes that spanned 360 million letters. Now we're up to 12% of all of our DNA.
And because of how they did the work, this is probably an underestimate. In other words, even more of our DNA probably has changes like these.
A lot of this repeated DNA includes genes. Which means people not only have differences within their genes, but also in the number of their genes. We are definitely not as alike as we once believed.