Eli5: What does percent in DNA mean?

Question

Siblings share 50% of their DNA. Then scientists say we have 1-4% Neanderthal DNA, but say we are 99% related to chimps. Then there is the fact that we share 60% of our DNA with bananas.

In: 18

Answer 1

They mean different things.

Think of your genome (DNA) as your house. To function, you need some basic things. All houses have (for example) a dishwasher, a fridge, a bed, and an oven. In that sense, you and your neighbor share 100% of the house contents.

But let’s say your fridge is made by GE, and has 2 doors. Your neighbor’s fridge is made by Maytag and has 1 door. In that sense, the appliances are different.

In that same way, we are 99% “related” to chimps in that we have mostly the same *genes*, just different *versions* of those genes. Pretty much all living things have an enzyme called hexokinase (just a random example). In that way, every living thing is related! But it turns out that while you have the same hexokinase as any other human, it’s not the same as the hexokinase found in a banana or in a bacterium. Same genes, different versions.

*Within* humans, (i.e. it’s already understood that you share all the genes but have different versions), you can also have variability. That’s why you don’t look the same as your neighbor or your neighbor’s neighbor.

For example: let’s say hair color is a single gene (not really but bear with me), and it has 2 varieties (called alleles): brown and blonde.

Pete and Bob both have the hair color gene. Now Pete has brown hair and Bob has blonde hair, so clearly they have different *alleles* but they both do have some form of that gene. Bill, Bob’s identical brother, also has the hair color gene but he has the exact same copy as Bob. So in that sense, all 3 share the same gene but at the same time, in a different way, only 2 of them share it.

Itsy Bitsy the spider, on the other hand, does not have *any* version of that hair color gene because she doesn’t have hair.

So “percentages of DNA” is unfortunately too vague a term to actually be meaningful on its own. You’d have to specify what you actually mean: is it just having the gene vs. not having it (as in the case of Pete and Bob vs the spider), or is it variations (as in Bob and Bill vs Pete)