Help! I cannot seem to grasp why siblings are said to be 50% alike genetically when humans are said to be, say, 85% the same genetically as mice.

-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.

Now, this is to any human. How similar are you to your parents? Or to your brothers or sisters? This is where the 99.95% comes in.

Your parents are 99.9% the same. You probably know that you receive half of your DNA or genes from each parent. So this means that you are 100% identical to each parent for each set of genes that you get from them. (Click here to learn why this isn't strictly true.)

Because your parents are 99.9% the same, any DNA you get from them is 99.9% the same too. So really, when we are looking at how similar you are to your parents, we only need to look at the 0.1% difference.

Now, this sounds kind of weird, but stay with me. Your genes are an average of your mom and dad's genes. So wouldn't we just average 100% and still get 100%?

No because even though you are 100% identical to either set of genes from your parents, your parents are only 99.9% identical to each other. This means you must average the 100% with 99.9% which gives you 100 + 99.9%/2 =99.95%.

You are 99.95% biochemically identical to either of your parents at least in terms of DNA sequences. It takes awhile to wrap your mind around this one unless you naturally like to play with numbers for fun. So that's looking at our biochemical identity by looking strictly at genes as a series of letters.

You may be thinking that must mean that my brother or sister is also biochemically 99.95% identical to my parents. So why aren't my siblings and I 100% biochemically identical and therefore genetically identical? Because you don't necessarily inherit the same 0.05% difference.

And this gets us to the next part of your question. In that 0.05% difference between you and your sibling is the 50% relatedness people are talking about. Huh?

The 50% refers collectively how much "genetic material" you get from each parent. You get 50% of your genes from your mom and 50% of your genes from your dad.

Essentially you have two copies of each gene -- one copy from your mom and one copy from your dad. Here is the strange part, the copy that you get from your mom may or not be the same copy that your sibling gets from your mom.

Remember each of your parents has two copies of most of their genes too. When the egg or sperm that made you got made, only one copy of each gene was put in.

The copy that gets put in is chosen randomly through a process called meiosis. What this means is that you have a 50% chance of getting one of their two copies.

That probability doesn't seem impressive until you consider that you have around 25,000 genes. Throw in a 50% chance of getting one copy versus your sibling getting another copy and that makes meiosis a serious gene scrambler.

So, because of this scrambling you and your siblings are 50% genetically identical and are not 100% biochemically identical.

You and your siblings are closer to 99.95% biochemically identical. Of course, since we have 6 billion bases, a 0.05% difference still translates to 3 million differences! Now explaining how people are so different and yet be 99.95% "identical" is another topic and we are just starting to understand this phenomenon.

By Dr. Aaron Shafer, Stanford University

More Information

You are 99.95% the same as your sibling.

We get half our DNA from dad and half from mom.