-A curious adult from Ohio

March 17, 2006

You have asked a very interesting question. It's easy to see why this sort of thing is confusing -- it doesn't look like it makes any sense! The first thing we need to do is separate the apples from the oranges so that we are comparing the same fruit so to speak.

When scientists say a mouse is 85% the same as we are, they mean that biochemically, we are that alike. Using this measure, siblings are actually 99.95% the same. The 50% refers more to the amount of DNA we get from mom and the amount we get from dad.

So we are more similar to our relatives than we are to a mouse. Let's explore in a bit more detail what the 99.95% and the 50% numbers mean.

For those of you not familiar with DNA, DNA is the alphabet that makes up the instructions for making you. DNA is a long string of four types of molecules known as bases. Bases are a type of molecule or biochemical.

These four types of bases are referred to commonly as A, C, T, and G. Some of you may have seen the movie GATTACA. The title of the movie is a play on the bases that make up DNA.

How do a bunch of letters make instructions? The same way they do when we write out instructions. The string of letters forms a code that our cells can read.

A single instruction is called a gene. A gene can have a string of DNA as small as the 24 base one found in the round worm C. elegans to a gene as large as the one in humans that is 104,000 bases long. The gene has the instructions for making a single protein.

Here is an example of a gene: "GACTTAGAGGTTACCTTACA- TAAGAGCCCTTTGGGACCTT." Just a bunch of letters attached to each other in a specific order. Of course each letter is actually one molecule attached to another molecule.

When scientists say we are 85% the same as a mouse, they are referring to the string of letters in a gene. This simply means that on average, if you compare a typical gene from mice to the equivalent gene in a human, 85% of their bases will be the same. Or if the gene was 1000 bases long, 150 bases would be different and 850 would be the same as the other. In other words we are 85% biochemically identical to a mouse.

How biochemically identical are we to our fellow humans? The DNA sequence in your genes is on average 99.9% identical to ANY other human being. Meaning, if you have a gene that is 1000 bases long, on average there will be only 1 base that is different between you.